Focus a key light on each soloist, or on each person in the band if the ensemble is small enough.
The key light is light from the front of the stage that highlights a performer. This is the main light that allows your audience to see the band. One light per soloist can be enough, although two is preferable. Focus a light on each side of the musician at equal angles to create more dramatic lighting.
If you are lighting a large orchestra or choir, you will not have enough lights to assign a key light to each person. Use several lekos, ellipsoidal reflector spotlights, for front light to create a wash of light across the band, and save the individual key lights for the solos.
Focus a back light on each soloist or performer. This is just like the key light, but from behind. This helps your performers stand out from the background. One back light will be enough.
As before, don't give individual back lights to everyone in a large band. Use them for the soloists.
Use color to create different moods. If you have a rock band, it is very common to use PAR lights to add some punch and color to your show. PAR is short for parabolic reflector. These lights are hung with gels -- pieces of colored material. Bright and rich reds, blues, greens and ambers are often used. These are focused on the musicians and the stage areas to create some interesting color washes.
Larger ensembles may perform in front of a cyc, short for cyclorama. This is a big, white piece of material that covers the back wall of the stage. Cycs are lit with strips of lights called cyc lights, with gels used to create different colors for the background.
Design your cues, or looks, for the show. This is where you can be creative, but keep a few things in mind. Keep a basic level of light on all of the performers, bumping up the keylights of the soloists as they play. Use your colors to punctuate the moods of the singers. Look for places to change colors and looks in the pieces.
Have fun. The lights can be as important a part of the show as anything else.