Well, if you have a sound tech there, they should be able to handle most of any volume concerns. If you're doing your own mix, then you may want to spend the time to do a really thorough sound check before hand.

General advice is to not "pop your p's" - in other words, try to not make any harsh pops or clicks (hard k or p sounds, strong sibilant s sounds, etc.)

In my experience, if you're mixed correctly, you shouldn't have to worry about how loudly you sing. You just have to worry about how well.

Hope this helps some...

M