The biggest mistake a band can make is to promote yourself to early. There is no going back once you have promoted your first gig, played it, and bombed. You have just started the word of mouth train, and in a cover band it is very hard to come back from a bad word of mouth review.
It so important to make sure you are "show ready" before you attempt to get your name out to the public. Know your songs, be comfortable on stage, and have your set list down.
I'll talk more on the importance of your set list later, but more importantly is your ability to be comfortable on stage. People can tell if your uncomfortable. It stands out. Of course you may be nervous.. I still am when I get on stage. But the imperative thing is to be yourself, and have fun. Why else are you on stage right? In live shows things will ALWAYS.. and I repeat, ALWAYS go wrong. What matters most is your ability to roll with the punches and make a good time out of a bad mistake or mishap.
If you get that "deer in the headlights" look on your face if you accidentally forget a lyric line, people will pick up on it. If you roll with it, and act like it was suppose to be that way, 9 times out of ten your general audience will never even notice. (Hell.. their probably ordering a shot, and trying to get that pretty girls number anyway). But if you stop singing, or make some big announcement how you screwed up, they will notice, and rest assure they will remember!
But the point of all this was to have a product to promote. Be prepared, Be comfortable, and have fun.. If you aren't having fun, neither will the people who came to see you. Remember.. these people took time out of there weekend to come see you. Make it worth their while, or they will never come back again, and chances are they will tell 10 of their friends about you. And we all know what happens with the word of mouth train.. It can make you or break you!