As the others have previously alluded, make sure that you are doing the sound check at the same volume level at which you would perform. Most people have the habit of whispering "1,2,3" into the mike while pressing their mouth up against it for the mike check, then when they go to start a song, they lean back a little and start singing much louder...their proximity and volume are vastly different and it makes the sound check totally useless. If you have a volume knob and built in electronics on your guitar, make sure the volume is turned up to between 1/2 and 3/4...this will send a strong enough signal to the board, but also allow you a little bit of room to turn up or down in relation to your voice. It is very important to find out if the monitor mix is the same as the house mix or if they are different. IF they are different you may end up wrecking the house mix if you change any instument volumes (including just singing louder)...instead, ask the sound guy to turn up the volume in the monitor mix. If both mixes are the same, then be sure to pay some attention to what it sounds like because everyone else is hearing the same thing...

Will there be a sound guy or are you doing this yourself?