It is a very real danger when you're playing a musical: Your brain playing all of the songs from the show on repeat, all day, every day. I have come up with a few ways to avoid this.
1. Once you start playing the show, don't ever listen to it outside of rehearsals/performances. I don't care if you just discovered an awesome recording, or if you have one that you've always loved. Don't. Do. It.
2. Do not hum or whistle. You will start humming or whistling tunes from the show without even knowing it.
3. Listen to other things - preferably other musicals - on your down time. Get these stuck in your head. Trust me, it will be better.
4. Start singing other songs - again, preferably showtunes - immediately after a performance.
5. Just give up. There is no hope.
Two weeks of The Fantasticks down and three to go! Happy September, everyone!