Keep your eyes peeled and you'll see the occasional vacancy on the Internet. If you have to travel far for an interview, be sure that it is agreed that your expenses will be covered. You can also make a post on Ajarn Forum to ask about the best and biggest employers in a particular town or city. This worked very well for me when I was looking for work in Khon Kaen.
General Internet searches tend not to provide a very complete picture of employers because many of them don't have websites. Those that do are unlikely to show any interest unless you're in the immediate vicinity. Like many other schools with websites, they too get bombarded with emails from would-be backpackers all over the world. For the most part though, you actually need to be here to find work.
Many larger towns and cities publish free maps that feature most of the leading schools and universities. Maps can be found in some hotels and many foreign bars and restaurants. Other foreigners can also tell you where you might find teaching work but, unless they've been here for some time, it's unlikely that their lists will be extensive.
When you're ready to look for work, hire a tuk-tuk [expensive] or a motorbike [inexpensive], put on your best glad rags and do a tour with your CV/resume [and copies of your certificates if you're going to universities]. It is always best to have a mobile telephone so that prospective employers can make follow up calls. Universities will only recruit people with verifiable degrees. The better private schools require two job interviews, several observational classes and a probationary period.
There are quite a lot of private language centres too. With the exception of AUA none of them can offer permanent work. If they tell you that they can, then the work is seasonal and will quickly dwindle away after the big school holidays. A certain chain of language centres will find reasons not to pay your final months salary. The simple solution is to not give them a months notice. Fair's fair after all.
Many private kindergartens are also crying out for foreign female teachers.
Some poor schools, particularly those with private benefactors, will pay about 20,000, whereas government and private schools and universities pay around the 30,000 mark. Some private schools also offer free limited accident insurance. Schools that also hire native Chinese teachers usually pay them by the class. If you are offered the same, demand a guaranteed minimum monthly salary or leave it at that. N.B. Chinese teachers also get free food, accommodation and return flights.
I'm sure that voluntary work is available, particularly in rural areas but having never done it myself, I'm not the best person to comment on it.
So, when is the best time to come looking for work? Anytime that there's a vacancy basically! You may find a headmaster in their office during school breaks but they won't be in a position to offer you anything until the new term starts.
The following can most definitely be intended as a word of warning. It has also been inspired by recent media reports and posts on Ajarn Forum. I know of two particular foreign paedophile cases here. One committed suicide in a police cell by swallowing his own sock. The other didn't get his picture in the local newspapers because his face was in no fit condition to be photographed.
As with elsewhere in the country, the police here also don't foot hospital bills for anyone who violently resists arrest or seriously injures themselves while trying to make a quick getaway. Read into that what you will.