A couple of tips once you have the gig. Make sure you are well rehearsed. Make sure you are tuned before you go on stage. Make sure you have another guitar tuned and ready to go in case you break a string. Make sure you adhere to the time constraints of the club by going on stage and—more importantly—getting off stage at the times the venue wants you to. Bring appropriate gear for the size of the club; don’t bring huge amps for a small club. Don’t be a prima donna with the sound person regarding your monitor mix; get it close, and live with it. Make sure you mention your name or your band name at the beginning, middle, and end of your set, and make sure people know you have a mailing list to sign. Thank the sound person (buying him a beer is a good way of doing this) and the booking person. In figuring out your set list, the rules that applied to picking tunes for your demo apply here. The ultimate goal is to get a record deal, so if you treat every show you perform as if there is an A&R person in the audience, bear in mind the notorious ADD issues of A&R people, and present your music in its clearest and most forthright manner. And, most importantly, make sure there is fog in the damn fog machine.
As you move from your first gig to gigging more regularly try to develop a relationship with a venue that is most appropriate for the music that you make and grow your crowd there. Loyalty breeds loyalty. This doesn’t mean you shouldn’t play at different venues around town, but you should try to have a kind of home-base venue. In this way, the venue will be rewarded for their initial belief in you, reaping the financial benefits of the big crowds that will come to their club as your popularity grows. Additionally, this will give you the opportunity to make multiple impressions on the people who work at (and frequent) the venue. It is these people that are in the position to help you with your career—either directly or by spreading the word.