Figure out how many artists are involved in the concert. Knowing whether you are working with a jazz trio or a full-blown choral group will help you determine how much space you have to work with.
Determine the set orders of the bands involved. This will make loading and unloading equipment easier.
Set up the amps. You can keep all of the amplifiers up on stage if you line them up in terms of set order. Place the amps for the first band playing in front, with the amps of the other bands put in the order in which the band will be playing.
Set up speakers on the stage. The main PA (public address) speakers should be placed toward the front of the stage on both sides. Face them at an angle to the audience, and elevate them on poles for optimal sound.
Set up the stage monitors. These allow the artists to monitor the total sound output as heard by the audience. Place one at the center of the stage where a lead vocalist would be. If possible, place more at the front of each end of the stage. It's good to have one available in the back for the drummer as well, because he has no sound coming toward him.
Set up the microphones. Ask the acts involved how many microphones they will need. Unless the concert is taking place in an intimate setting, you need to mic the guitar amplifiers. Place the microphones in front of the amplifiers. Point them toward the speakers but keep them at a distance so as not to get feedback and distort the sound. You can run run a direct input from the amp into a mixing board. Place the amplifiers at the side of the stage at a 45 degree angle, so as not to assault the audience with guitar. Mic as many pieces of drum sets as possible. If you only have a few microphones available, just mic the kick and snare drum. Don't mic the bass drum.
Set up lighting, fog machines and other equipment, according to the needs of each artist.