Man, if you knew the always right answer to that one and could bottle it, you'd make a fortune. You'd also keep a million performers from driving themselves crazy with the "what-if's". Some people think that you should work out a formula and run it every time. Up tune, funny tune/sing along, sensitive tune, change of pace (blues, maybe). Some people even work out the succession of keys in which the songs play. I tend to think that you can throw away a lot of good songs, early in the night, if you're audience isn't ready for 'em. Gary T's advice is pretty good, but what it focuses on "you" - the performer - and forgets about "us" - the audience. Performance is part give, part take; even though you did your best song absolutely flawlessly, it doesn't matter if it fell on the proverbial deaf ears.

I feel that two key elements in how you start out are 1. The type of crowd you're playing for and 2. what you're trying to accomplish. In my experience, the best bet is to be prepared with a couple of alternatives and then try to gauge the mood in the room. Although "UP" tunes can generate a lot of energy, be sure that's wher you want and the crowd want to go that night. You can alienate the crowd by being too big (too loud, too intense, too damn cheerful)for what had been a pleasantly laid-back group. You can also be making a promise you ain't going to keep if the rest of your set is in the sensitive ballad vein.

What seems to me to be most important, whatever the specific song you choose, make sure it's a song you feel confident about and that you really, really like tonight. If you're not confident about playing it, your audience will sense it and they will feel as uncomfortable as you do. And if you don't relly feel like playing (or hearing) a certain song tonight, why think that the audience will? Just pick one and throw it out there and if it doesn't work go get them with the next one.