It takes alot of time to find the right tailor, trust me. I use suits everyday of my life and for me, to find the right tailor is very important as I have to update my wardrobe regularly. True, there are amass of tailors in Bangkok, but not all of them are as great as they seem. C'mon, a suit, 2 shirts, extra pants all for $199? That's by far the most ridiculous thing I've ever heard. Those deals are just too good to be true. Do not even bother going into any shops that offer those too good to be true deals.
Secondly, do not bother going into shops that have touts standing outside trying to pull you in. If I wanna enter the shop, I will. No use trying to drag me in. The more they do that, the more I don't enter that shop.
Lastly, enter a shop that recommends fittings as part of the process. If they don't and manage to convince you that they can finish the suit in 24 hours with no fittings, please do walk out. You do want the suit to last more than a few months.
As a personal recommendation for a shop that has consistenly produced good suits for me without offering any of the above crap is Crown Tailors. They're inside of Sukhumvit soi 8. Small, well lit and easy to find. If you want some good suits, there's your place to hit. This isn't one of those hit and miss places as I no longer look around for tailors because I'm very happy with the production of Crown's.
Happy Shopping everyone!