We'll address restaurants, taxis and other service people another time, but here are my tips on tipping where you hang your hat while in Thailand...
Let's start with something fairly consistently agreed on: tipping in Thailand by the Thai is not the norm. The whole concept of tipping came from tourists tipping people, and in some ways we've created our own problem. It's not an area where you're liable to make a huge social faux pas, but there are a few things you might want to be aware of. These aren't facts carved in stone, but I don't think there are many who would disagree. If someone does, I hope they'll leave a comment.
First thing to remember: don't grossly over tip. Extreme over-tipping can make you appear to be a show-off, a fool, or both to the Thai. While it's nice to be thoughtful, lavishing large gratuities is looked upon as condescending and isn't respected. Oh, they'll take it - but you're liable to lose face in their eyes if they figure you're patronizing them, which most likely isn't your intent.
Second: Many restaurants, hotels and clubs tend to pool tips together and share them among the people involved. Some places will have a designated tip box at the reception desk, the cashier's station or some place visible to the customers. There's no real way of knowing who it's doled out to, or if it's being distributed at all; you're flying on trust here, so decide for yourself. I feel most certain about it if I tip the person directly, but that's neither possible or practical in some cases.
Hotel bell boys: I tend to tip 20 baht per bag if they're using a cart and an elevator. If someone's lugging three heavier bags up two or three flights of stairs I double or triple that, myself; you have to make a judgement call, really.
Hotel housekeeping: Your room is your temporary home, and you'll probably appreciate it if it's kept properly. Larger hotels usually work their people harder than a bed and breakfast, but not always. Either way, it can be a difficult, unpleasant job and someone who takes good care while doing their job is worth regular recognition. Personally I leave 40 baht a day, either on the unmade bed or on the desk, with a note saying "Housekeeping - thank you", in Thai if I can.
Again, I prefer to get it to the person who's cleaning the room, so in a larger place I'll sometimes ask whoever's at the cart in the hall if they have my room. If they do, I hand it directly to them.
Other hotel employees: If someone does me a favor, I feel they deserve something more than a mere thank you. For example, if I've made a foolish mistake with the room safe and someone has to come re-set it for me, I tip. If I've dropped a glass in the bathroom before dinner and ask to have the shards and slivers cleaned up right away, I tip.
Treat others as you'd like to be treated, I say. Someone isn't "below me" merely because they do a job I'd rather not do myself. What goes 'round comes 'round, and besides - out of the total cost of the trip, it's next to nothing.