Don't worry; I am sure this happens to all of us tile pros. The trick of course is to cut out the hole while leaving the drywall backing in place. Here are some things that I have learned over the years that helps me:

1. Open your razor knife partway so that it goes in a metered depth (apx 3/8") and cannot go all the way through the drywall.

2. Keep the claw of your hammer sharp using a grinding stone so that it cuts right through, rather than simply knocks into the drywall. A sharp claw also helps to better scrape out the excess drywall material.

3. If you do make complete hole in your drywall, simply stuff it with newspaper, plastic dropcloths, or whatever else is handy, so that you have a backing for your mud.

4. Don't forget to carefully puncture a relatively small hole in the backing with your knife to allow the mud to go through and mushroom out.