Some sounds advice:
-Be comfortable. Comfortable shoes especially. Your feet will hurt. The first few times to your legs will be sore (depending on how long the gig is) until your muscles get used to it. Once the adrenaline wears off you'll notice it.
-Act as if. Mistakes happen - it's how you rebound from it that is important. If you flub a note or a part, blow through it and don't show the "oops" face. Most people have no clue.
-Check and inventory your gear before you leave for the gig. Getting there just to find out your missing something is horrible... especially if you don't have time to go back.
-If you have an extra bass, bring it. If your main bass dies you can just grab and go.
-Lock in and GROOVE with the drummer - don't over play. Concentrate on the groove instead of the extra flashy stuff. You're what keeps the dance floor moving.
-Have fun!