Set up a feedback monitor so that you can hear yourself sing. Check the levels before the performance with all instruments playing that you'll have accompanying you during the show. Make sure you can hear your voice over all the other noises.
Make sure your instrument and your band members' instruments are in tune. Once everyone's in tune, give yourself a reference point before beginning each song. Play the first note of each song and hum the note to yourself. Hold onto the note in your head until it's time for you to sing it out. Starting on the right note is essential for getting the rest of the song right. Over time, you'll find ways to do it almost out of habit that go with the natural flow of your performance and even allow you to talk in between songs.
Place one hand over one ear or close the ear canal with a finger to hear your voice during the performance. Listen to your voice as you sing. You can do this at moments when you don't have to play your instrument. If you don't play, don't leave your hand on your ear the whole time. It doesn't look professional. Just check in once a song, especially at those points where there are key changes or notes at the edges of your range.