What I like to do is bring a book with me or homework, that way I can order my tea and focus on what's in front of me rather than worrying what people think (because I know how you feel). It also gives me a chance to relax and take my time. I wouldn't give them an extra tip if they're rude, unless you are taking a very long time. Just smile and be happy and they should be happy back. I've never had any experience with rude waiters, perhaps try a different restaurant.