Choose one:

1) Turn the vocals up.
2) Compress them (search for "vocal compression" on this site, search link is at the top)

When I run live sound I find that vocals disappear very easily unless there's more of them. When 5 people sing it's easy to keep them above the level of the instruments without causing problems with feedback.
You could try a chorus effect but it's probably not desirable.

(continuing from codemonky's numbering)

3. Carve a spot for them with the EQ.

You can find where the most of the vocal tone is serveal ways:
1. you can use a spectroscope/real time FFT.
2. use high and low pass filters and move them to find the majority of the tone that carries the voice. Don't use this as the final vocal EQ, but cut these frequencies from the instruments.
3. bus the instruments and cut out some of the middle until you find a range that makes the vocals come forward.

female is typically 200-7k
male typical 100-6k

These will vary depending on the voice, and what they are being mixed with.

EQing the voice will quickly make it sound unnatural, so it's normally better to cut the instruments than to boost the vocals with an EQ.

compression definately!!!

and if you have a GOOD eq.....push 7-10 k to make it sparkle (after comp)

this will sound crap on a bad eq

pan other inst out so vocal has a spot in the middle

drop other inst in the 3k area---this is where the vocal has presence in sharp sounds

----this is my first post here by the way

Pan the vocals to the middle.
Pan Bass and Kick a little to each side of the middle.
Pan everything else away from the vocals.

+ what everyone else said.

use a limiter like l1, l2 or l3 (my recommendation would be in the reverse order) to alter the sound and loudness to just the amount that you find desireable. if you have recorded your vocals on a single track you will make them sharper by slightly limiting them with these processors. of course they can get really sharp if you go all the way...use with caution but definitely an easy way to make your vocals punch through the mix provided they have been recorded pretty much in a linear fashion...