-There are a million players out there that are as good as you or better.
-If you are lucky, you fall in with the right guy that knows a guy.
-You get a gig doing whatever and they ask you to do whatever.
-You are able to handle whatever is asked of you.
-You show up on time and prepared every time.
-People have an overall pleasant experience working with you.
If you goof up any of those three things (and you gotta get a lot of mileage before you qualify for any second chances) then next time they will ask one of those million other guys that are as good or as better than you.
Oh yeah. And business cards. Nobody will take you seriously if you don't have business cards, even if they are a very simple design. Also having a professional email address is important. mike.thomas.89 or jsmithguitar are good. rawkgawd666 or the.dark.lord are not good. Name, numbers (whether birthdate-related or arbitrary are good; 666 and 69 and 1337 are bad), and basic keywords like guitar, drums, lessons, session, etc are all good stuff. Words like evil, crazy, god, demon, dirty, etc are not good. This is a professional sounding email, not a CB handle. Some diminutives are fine. Steve, Danny, Bill, etc are okay but something like Megz, Jim-Jam, or Jeffay would be bad. Also using a current email domain like gmail is preferable. Using AOL or CompuServe or Juno makes you sound out of touch.