Band Fight!

I think pretty much everyone has this problem once in a while, if not often.

I like practicing with headphones - if you've got the resources, throw a mic in front of each guitar cab/bass amp, one for the singer, and whatever you've got left on drums and run it through a mixer into a headphone amp. It might not be pretty, but you'll hear what would be pumping through the PA at a gig and it might even push you to play better when it sounds like crap. You also don't have to worry about who's louder than who, just turn it up on the mixer.