I tip around $10 whenever I'm at the clubhouse for a few hours and receive excellent service. Every so often (very often, actually) I'll have a server who is absolutely amazing -- fast, fun, and understands that they don't need to ask if I want another drink. It makes for a perfect pre-flight afternoon, and I feel like the least I should do is to leave a small tip as a token of my appreciation. They're always extremely grateful, too.