Always be prepared by having the proper tools. Have several guitars and amps, direct boxes etc...for different types of music and carry other instruments like banjo, mandolin, bass etc... just in case they want a specific sound or another musician doesn't show up.. (if you double up on more than one instrument you get paid twice too)
You've gotta be able to take handwritten music with scribbled out changes and be able to read it cold.
You also have to have a good ear for improvising... (sometimes a part will just have a line through it that says 'guitar solo... country style'.) Be able to follow a conductor, and be willing to swallow your pride if you don't like a part but everybody else says 'that sounds great'
A lot of the business is word of mouth too so on your way up in the business, you've gotta be business savvy as well as be a salesman. Make sure you never screw anybody out of a job or act like a dick, cause fer sure the word will get around and those who've been around a few years will quickly spread the news through the grapevine. (pay your dues)
Lastly, if you don't live in a major city, ferget it.
You've gotta be right where the action is and be ready to get to the next gig day or night, or 'right now'.