A band is like a family. A good bassplayer in the band is not necesarrily the best bassist that comes to the audition. They will pay attention to more aspects.
1- be friendly. Thay are looking for a family member, remember that, so they are looking for a guy to feel good and have a lot of fun doing music.
2- do't be afraid to ask before the audition what do you have to learn. Ask for MP3-s or CD-s if they are an original band, etc. It is better to know 2 songs good than 10 songs bad.
3- do some research before. Ask about them look to the internet, find informations about them etc. They will be very well impressed if you know their names, their songs, a little bit of history, etc. It is much better than : "what music are you playing guys?"
4- like Kris said, a good image will help. Now ... a good image for a Jazz band is a little bit different than a good image for a Punk Rock band. So... try to be in the same picture as them.
5- be at least 10 minutes earlier. You don't have a chance if you are late.
6- don't show them all your technique, they will not be that impressed. They will look for sound, ballance and knowledge. If you go as a bass hero, they will tel you that you should have your own solo album

Don't try to impress, a musician will "feel" your value at the first notes anyway, you cannot make yourself to look like a better or a worse musician than you are.
Good luck, and remember, probably they will choose the best guy to drink beer with instead of the best bassist.
So just relax... music is beautiful
