Look happy and enjoy your music (or whatever your act is).

Try to keep "up" tempo - cheerful patter, a well seeded instrument case or whatever. Some people take pleasure in trying to get a coin into your case/box from a distance - put your coat or jumper over to stop the dosh from bouncing off - I once actually drew a target on a piece of paper and sellotaped it to the open lid of me case, and it worked a treat! DON'T upset drunks, youths or half wits, a pleasant but firm "i'm trying to earn a crust" seems to work - though not always.

Best of luck.