At HGI, I follow the following practice:

1. My base is $1, if it is a buffet and I serve myself -- as long as they address me during the visit.

2. I leave $2-$3, if they actually are friendly, ask me if I need anything (e.g. Tabasco, ketchup, refill of juice or coffee), etc.

3. Very rarely, I leave nothing -- if they don't provide service or are not at all friendly, dirty tables all around me, etc.