Forgive me if I tell you things you already know...
Need more info. Has your vocal mix ever sounded good? How does it sound with no instruments or drums playing? Is the bad mix specific to the room you play in - does it sound better in other rooms? How does the mix sound if you play outdoors? How does the mix sound in closed (over the ears that block out external sound) headphones?
I am not a pro sound man by any means, but I have run a bunch of PAs over the years; I am sure others here are more knowledgeable, but I'll toss in my $.02 anyway...
The room you play in has a huge effect on what you sound like. That's why I asked about other rooms, outdoors, and the sound through headphones. Eliminating the room as the cause of the problem by playing somewhere else or outside or listening to the PA only through cans is something to try. The room I play in got better sounding after I built some baffles. See my Rehearsal Space thread, for more info. May not be the right answer for you, but couldn't hurt.
I like to start off playing "dry", that is no eq or effects on the voices. I only add reverb or echo after I have done any eq, because those effects make it too confusing otherwise.
I haven't tried this, but a friend of mine swears by the Behringer DEQ2496 EQ. It has an "auto eq" feature that runs tone through your speakers and eq's the room correctly. Found a couple of links:
http://www.enjoythemusic.com/magazine/e ... ve2496.htm
If I had a difficult room, I might try something like this. Under $300.
Sometimes, just not playing as loud in a room can help the sound.
Such a long, long time to be gone, and a short time to be there...