Regarding setlists, I always felt that the 1st 4th and last 2 songs were the most important.
My thoughts are somewhat similar to Toms:
First song is first impression to anyone in the club that's never seen you. Should not be the big "hit" of the band, but should be IMO one of the best representations of what you do. It has to kick ass, but should also not kick the most ass, as you still need to move up from here and keep people's interest. Also, something The Nerve used to do (pre first song) that worked incredibly well for the type of band that it was, was to actually start our shows with some ridiculous 3 part acapella thing. We had many of them, most TOTALLY non sensical. One of the ones that made more sense opens this video
http://www.youtube.com/joenerve#p/u/6/r8wuMyKsTUw . It was insurance that we had everyone's attention before we kicked into the first song. And it worked.
Fourth song I always used as a changer upper. By the end of the third song I think an audience likes to know you can do something different, and that not all your songs are going to be the same vibe. Kind of re-hooks them in, and may grab some others if they weren't into the vibe of the first 3. Also gives a little bit of a rest.
Last song is a no brainer. I always wanted people leaving the club humming our tunes. Whatever we considered our "hit" they'd have to wait for. If there was no "hit" then it was our catchiest most memorable bestest tune!
I said last 2 songs above because ya always have to make provisions for encores, and for the last song being cut by the club. Which happens. Often. Last 2 often have to get juggled and need to be well thought out.
Almost forgot... another thing I consider key. I always like to make the 2nd or third song a cover. When a band is brand new to an audience I think it pulls people in, gets them on your side, and keeps them hanging around possibly hoping for another, which we rarely if ever gave em. Audiences like to have something to identify with.
Last note on this. Unless you have a huge fanbase, short sets are always the way to go IMO. A half hour set leaves people wanting if they like ya, and saves people if they don't. I think it's about making a mark and then moving along. The 2 1/2hr. gig will make anyone sick if it's unkown music, I don't care how excellent a band it is. The 1 hr set is enough to start pushing people out. I like 1/2 hr sets, 45 minutes tops. That's more than enough to give em all you've got.