We do not tip in advance and found our service from everyone (cabin attendant, waiter, ect.) to be exemplory! I think what helps is to be very polite when requesting something, saying how much you appreciate their help and treating them like ''people''. We listen to their stories about their families back home, ask a couple questions and empathize with them. These people work very hard and very long hours, and are very eager to please. I think they will provide you with the extras wether you tip at the beginning or at the end.