The traditional Thai salve/vale/mea culpa is the wai, made by placing the palms together in front of the thorax with the fingers pointing skyward. (The same gesture is a nop in Laos.) A small accompanying bow seems to be common, although not obligatory. Change the height of the hands and it changes the status of the wai: there's the rub.
It’s charming to be waied after a short conversation or transaction (I may sound all memsahib, but it really is the most charming greeting gesture I can think of). While in the retail paradise of Siam Square, I didn’t generally respond, usually because I had my hands full. But the guidebooks say you should reciprocate. More recently I have responded; it has felt a little forced and I’ve got it over with as quickly as possible. But it gets a little easier.