The above advice is right on. Here are a few lesser points.
1. Carry mints, combs, brushes, shoe polish, all that kind of crap with you to every show. Dress like a pro and always look like a pro. Furthermore, a new deck for every gig please; no ratty sponge balls; no stained close-up pads; steam those silks, etc.
2. Bring business cards with you and don't be shy about handing them out.
3. Unless the show is a formal close-up or parlour show where you can control the environment, always be prepared to perform under all conditions: surrounded, standing, sitting, with table, without table, etc.
4. Have a close-up act, stage or platform act, walk-around set, kid show, family show, night club show, street magic set. Just be ready to say "yes" to any good money offer.
5. Don't underprice yourself (I admit to having trouble with this one sometimes).
6. Make sure you make yourself known and direct at least part of your performance to the client, client's boss, client's family, client's kids, etc.
7. Start performing a little early, leave a little late, give them more than they expected.
8. Don't drink at that corporate hospitality suite, don't try to make a date with the hot chick from marketing, don't [censored] where you work.