Once in Thailand, I recommend Thai Air Asia for domestic travel. Price is low, service is great, & they have a good reputation. Most would warn you to stay clear of Orient Thai, for safety concerns. I've used virtually all of 'em & have never had any issues.
Ooh... something to think about. Unlike how we, as crew, can travel through airports in the states & EU countries in uniform & be able to take things through security that passengers are not able to do(like liquids & gels), the Thai security at both BKK & Doung Mung(sp?) airports don't recognize that exception. Unless you are just lucky.
I do the trip roughly every other month. Unfortunately, usually at my own $$s. If you learn any sure-fire way, then do please pass it on.
Good luck. Have fun with it.