Smile of humour or happiness? If congruent with member behaviour, a genuine compliment is always welcome, personalised even better.
When I say congruent, the person is treating me with respect, not over stepping and not appearing ungrateful. Example; coming into my room several times a week for 6-12 months pouring compliments but never spending, even worse if they try to instruct me, and or make it clear they are wanking getting off without spending. I don't mind the getting off, I do mind it obviously being pointed out.
Genuine, not over the top, without pressure or instruction, a tip is a bonus and I make notes on this so I recognise the member next time.