I once saw such a sign in a tourist helicopter at Niagara. It simply said, "Tips are appreciated." Whatever wording you choose, don't make it too wordy. Too long a sentence might give smart Alecs a chance to say something that they think is clever, but nobody else does. For example, if you write, "If you want to make the driver happy, leave him tips," the smart Alecs could jump on that and have a laugh at your expense. I'm sure that you could think of how they might do that. Keep it short, keep it simple.