1. Dress nicely. If it's for retail sales you don't need a suit/tie but dress pants and shirt are a must.
2. Bring mints, Gas-X (an anti-gas pill), Tylenol, with you. You want to make sure you don't have a headache, bad breath, or a case of the farts before you go in.
3. Make eye contact. Don't look at the floor/ceiling.
4. Get there 10 minutes early.
5. Come by yourself. Don't let your parents, siblings, tag along. Or kids if you have them.
6. Answer questions honestly. If you lie they'll find out eventually.
7. But don't be afraid to BS a little. As long as you can say "I told the truth" afterwards.
8. Be confident but not cocky.
Good luck.