Bellhop - $2-$3 for the first bag and then $1 for each additional bag. I don't travel light, and the poor bellhop is usually drenched in sweat by the time he's done unloading my luggage, so I feel it's well deserved.
Buffets - 10% always, but I do tip 15%-20% if I have ordered an alcoholic drink that was not included in the buffet
Restaurants - I refuse to tip for bad service. I am very mean and will leave a penny or coupon so the waitor does not think I forgot the tip either. As for good service, it can range from 20% - 150% depending on the situation and the meal. If I have gotten decent service and my breakfast was $3, I certainly don't mind leaving more than a $3 tip.
Here's the reason why I favor a $0 tip rather than boycotting a restaurant: I feel it is more the waitor's fault than the restaurant's. Sometimes restaurants make bad hiring decisions. People can be polite, well mannered, etc. in an interview and turn out to be a terror on the floor. So long as the restaurant fires this rude waitor, the restaurant has done what is necessary to correct the problem. By not tipping, I am "punishing" the person who is problem while still supporting a restaurant I like.
And before I get flamed, I should explain why I would leave a $0 tip. (I have only done this once.) I was at a touristy restaurant on the pier in San Francisco with 3 friends, and we were starving! It was somewhat like an Italian sizzlers - you could order from the menu or choose the psata/pizza buffet. When the waitor came over, he asked if we were ready to order, so we looked at each other to confirm that we all knew what we wanted. In the split second it took for us all to make eye contact, he asked (in a very sassy voice, "What? You don't speak English?" All of us are Asian-Americans, mind you, and he had taken our drink orders in English prior to this comment.
We all ordered the buffet and when he brought us our clean plates, he dropped a piece of dressing-covered lettuce onto one of the clean plates. Yes, dirty food from another table's cleared plates. Instead of bringing a new plate as we requested, he used his bare fingers to pick up the piece of lettuce and declared the plate good as new. What is WRONG with some people?
In additional to this, we received all-around bad service. Waitor was slow, didn't check up on us (although this was probably a good thing), no drink refills, slamming things down on the table, etc. There were several tables in our section that simply got up and left without eating.
Needless to say, we didn't tip. We did leave him a nice grocery store coupon good for $0.20 off Pampers.