Clothing - less really is more
Bring professional clothes if you will need them
If you will be working as a teacher, women should make sure they have several sleeved blouses and men should have collared shirts. Occasionally, schools have a dress code that requires teachers to wear a particular color on a particular day so try to diversify. Women would be better off bringing skirts that go below the knees vs pants as many schools require a skirt and men should bring full length pants and ties.
A friend told me this before I arrived: 'imagine you are living in America's 1950's era' and you'll make the right fashion choice. If the dress code is more relaxed, it is much easier to find more modern styles in markets at a low price. Worst case scenario is that you have to pay mall prices for a dress shirt in a pinch. Most larger cities have used clothing markets too, so if you have time to get acquainted with an area before you start work, this can be a nice option.
As an American, I would say we tend to handle heat by stripping down to as little as possible, but if you look at some of the clothing choices in the hottest climates, covering up is a better option for staying cool and protecting your skin from the sun and mosquitoes. While plenty of people wear tank tops and sundresses in the bigger cities (and having a few would be beneficial), it is best to bring more clothing that has a minimum of short sleeves. As a foreigner, most Thais will not be too moved by more revealing outfits, but it is not likely to help you in any respect, from comfort to assimilation.
With regards to fabric, think lightweight cotton above all else. Linen is alright and denim is surprisingly abundant here, but synthetics make you sweat and they're overly abundant here. Having a single cool-dry style shirt might be nice for days where there's potential to get wet or for exercise (you know yourself and how much you exercise), but beyond that, cotton is king.
Thais are generally smaller, both slim and slight in stature, so those of us who are not both of those things will have trouble finding inexpensive things here. Other features, like ladies with curves or men with broad shoulders will also have a harder time finding clothing. That being said, you can find almost anything for a price and with some persistence, so don't feel that you have to pack for every possible scenario.
Length of stay and location
Pack light and smart
It may seem a bit counter intuitive, but I would suggest that most travelers who will be in Thailand for two or more weeks should pack roughly the same amount, with consideration for what they are doing. Finding the perfect shirt or skirt may take 10 outings but markets are everywhere, so if you have time (and you do not hate shopping), save some space for your new finds.
Think about what you would need over two week's time doing whatever you plan to do and that should get you going while leaving some space to acquire things here. Laundry facilities are abundant and inexpensive and, despite the heat, air conditioning is easy to find. The country also receives a fair amount of rain (for short periods) throughout the year, so overhangs and shaded walkways abound and most folks will not sweat through their clothes as easily as one would anticipate (you do adjust and it helps if you try sleeping with minimal air conditioning or just a fan).
Big cities abound with choices but consider packing more for rural areas
Location will have an impact of how easily you will find clothing. In a city like Bangkok, malls are abundant and have more selection than most western malls (however, prices are also comparable), but in the sticks, you will be looking at less size and style selection.
Consider what you can spend
Packing lighter is always to your advantage for getting around, but if you're traveling on a budget, it may be better to look at thrift shops or shop sales at home (especially if the factors mentioned above about size/location apply). The flip side of the coin could very well be that you become stuck devoting much of your free time searching for something here in Thailand and then have no negotiating power because you desperately need it.
That said, shopping is an adventure here, so that may not be unappealing to everyone. I arrived under the impression that prices were drastically less, but the big catch is that low prices apply to factory defects, imitators and poor quality clothing. This clothing generally sells for $3-10 USD and often does not have tags that denote the size, which becomes difficult at a street market where 'changing rooms' do not exist.
Toiletries - What is worth hauling across the world?
It should go without saying that time is a huge factor here in that short trips should logically include most everything you would want on any other short trip. Go light on the makeup unless it is waterproof and you cannot do without, but otherwise, the basics will do.
For the longer hauls....
What to Bring:
I read that deodorant in Thailand contains bleaching agents because light skin is very desirable but I was left thinking that I just needed to be sure I did not choose that kind. I will clarify here that 99% of deodorant here has acids and bleaching agents that are terrible for your skin so try to bring your own. They also carry very few 'stick' style deodorants and the one brand commonly found is $3 USD for a travel sized stick. Moral of the story, try to bring your preferred deodorant if at all possible, but you can find alternatives for a price.
Despite the value in light skin, sunscreen is difficult to find. ‘Boots' products are abundant here so you can find $20 USD natural sunscreen or a small $10 USD Banana Boat tube but that is about all. This just means you should consider if you burn easily or are sensitive to certain chemicals in topical products because most products have limited English ingredient lists unless they are imports (and even then the sticker is usually covered by another sticker written in Thai).
What to consider bringing
Medications you frequently use. Many prescriptions are available over the counter here for good prices, but if there is something you use on a semi-regular basis, just bring it along in case it is difficult to find something comparable, especially if you need a non-generic version. Any of the big drugs, like lipitor, viagra, valium, Yaz, Yasmin, etc are easy to find.
One good example of something a bit trickier to find is a digestive aid. Pharmacies are abundant and Pharmacists generally speak English well and can make recommendations for anything else. Don't bother with things you are unlikely to need like malarial drugs unless there is only one kind you can take due to an allergy (or you're looking to use a prescription plan that discounts it, or you're going to nearby jungles... you catch my drift...). When in doubt, it is generally possible to look up pharmacies near your destination via skype (or other international calling options) to make sure they carry what you might need.
A travel towel
There are plenty of average towels around, but if you are trying to keep things light, travel towels are great for their compact size and quick drying, especially when you shower twice a day.
If you are a regular user of lotion, the problems with bleaching agents in deodorant especially apply to lotion, so bring your own. I use straight coconut oil, which is easy to find here.
You can find these in Thailand, but they're no cheaper so it is probably worth starting off with a set to freshen up until you have found your way around and stocked up again.
If you are particular about your cu-tips, maybe bring some because the type sold here are not of high quality (then again, they aren't going to get stuck in your ear). I mention this because it is a complaint I have heard more than once.
Anything you prefer to use that is chemical free or all natural
Those labels do not carry much weight here and it is quite difficult to find organic or chemical free products (with the exception of places notorious for attracting a more environmentally savvy crowd, like Chiang Mai and Pai). If you are sensitive or have a strong desire to protect your body from harsh chemicals, you should bring those products with you.
They are more difficult to find than pads, so it might be worth starting off with a good supply until you find a place to stock up. I personally recommend looking into a DivaCup if you have not already heard about them.
What you don't need
You will find this commodity at every convenience store for a cheap price. While I question it's effectiveness at times due to the relentless bug bites I find all over my ankles and feet, most of that can be blames on the sweat that washes it off. Pharmacies hold the hardcore DEET products behind the counter but many 7-Elevens sell 20% DEET and Citronella variety bug sprays.
Bring a base supply of hairbands and maybe a travel brush but you will find plenty of that here
Fortunately, these products are not bleaching and they carry most of the brands you find elsewhere, so just plan to buy a big bottle when you arrive
Unless you want a 5 blade mammoth with soap attached, you can find this item most anywhere
Toothbrush and paste
I am not suggesting you not pack a toothbrush at all but rather suggesting you just pack a travel sized tube of paste and 1 toothbrush because these items are easy to find when it comes time to replace what you have. The only exception would be if you require a special kind of paste. Colgate is common here.
Other supplies that will serve you along your journey
It sort of goes without saying but, to clarify, bring a camera if you have one. If you don't you could probably find a good deal at one of the markets here. Ultimately, it depends on your feelings about photographing your adventures, but I recommend a small point and shoot. They might not always be the best photos but you'll get a more even spread of your experience because it will be easy to take with you everywhere.
If you have one that is not terribly valuable, it is worth bringing along as internet cafes are starting to become harder to find (Generally, they are marketed as gaming cafes with higher powered computers). The prices are comparable here with the exception of Macs, which are more expensive. and the choices are still good, but they are not always the same models as back home. That being said, having the Thai characters on your keyboard might come in handy while you're here and function as a nice reminder when you return home.
If your phone takes a SIM card, it is worth bringing along. Plans are cheaper here and a local SIM card is $50 TB ($2.75 USD). Phones are generally just as expensive here and certain phones, especially new and highly sought after phones like the iPhone 5, approach $1000 USD. Used phones are abundant and if you're looking for something basic, that's the way to go, but most everything here is buyer's beware; you buy it, you keep it, no refunds (some places will exchange if they have something else equivalent but don't count on it, especially if you don't speak Thai).
Books or Tablet
You can find books in English here, but you will likely pay full list price. Truthfully, for the hours of enjoyment, it is still not expensive, but I found it was worth the weight to bring along specific books because the selection here is not 'Barnes & Noble'. If you are not a big reader, you are unlikely to become one here, so spare yourself the extra weight. Cities like Chiang Mai do have a pretty great collection of books and Bangkok has a large community of expats that likely share books.
Tablets are comparable in price should you plan to buy one here, but if you have one already, you will find tablets that connect to WiFi or 3G immeasurably valuable.
Good walking shoes
Most will tend towards skipping the sneakers, so find some comfortable slip-on, more minimalist, shoes to bring along. Shoes here are hit or miss for quality and larger sizes (above 7) can be hard to find, so bring several pairs of your own. Often, shoes are left outside the entrance of residences, so bring something that is easy to take on and off and avoid something flashy (disclaimer: the chances of your shoes being taken are very low).
Backup storage via flash drive or memory card
This is easy to find in any city with a mall or bookstore but it can also be one more thing to find when you get here. I suggest having at least one storage device with you.
If you have one, it is definitely worth bringing (unless you plan to stay in a nice place and run the A/C every day; no judgment). They can be found in pharmacies here for about $10 USD. For the nominal weight it adds, it seems worth packing and avoiding that expense, but don't worry about buying one ahead of time unless you won't have time to stop at a pharmacy before you head into the jungle.
Flashlight / Torch
Unless you have a really nice one, you can pick up a simple one at a convenience store here. I mention it because a headlamp might be handy depending on what you plan to do. It made the list because it is something I forgot and there have been times where I wished I had it, like night rides on the scooter or bicycle. Otherwise, it is something you can easily find here.