I think having quality in every set is important. You need to do your research on the type of people that frequent the venue. By "type" I mean, age, gender, are they locals or tourists, etc.
What kind of music is generally played at the venue? Don't expect to keep a crowd in a country bar if you are going to play metal all night or vice versa. You need to find a venue for your type of music or change your music for the venue. Those are your only real options. Feel free to try a few different things if you want, just don't over do it.
I guess the four hour gigs are standard for me even though I do not do the bar thing much anymore. Weddings and festivals are still four hour jobs.
You can do four forty-five minute sets or try three one hour sets with two twenty minute breaks. Be prepared to do either if it's possible.
Be prepared to change your material on the fly. If a bachelorette party comes in and your next song is For Whom the Bell Tolls, you better be ready to drop it for Brown Eyed Girl, if you know what's good for you.
You never know who is listening to you in the audience. Some of my best gigs came from crappy nights with only a few people to play too. That one guy or girl may be on a committee for a local town festival or they may need band for a wedding. Always play your best and have fun with the people that are there to see you.
Have good break music. Give them something slightly different then what you play. Don't be afraid to play the new hits. Chicks like that stuff. Also, through in some line dancing stuff. There is plenty in that crap in every genre to pick from.
Go talk to people during your breaks. People love that. A simple Hey, or Howdy or Hello goes along way. People like other people who are not stuck up.
Just a few random things I thought of. In no particular order of importance.
Good luck!
