How do you drill into wall tiles without cracking them?

The best method I've come across is 1. mark the precise center of the hole you'd like to drill. 2. take a hardened nail, or a nail set with a 1/32" or less head, and tap it gently with a hammer to just break the surface glaze of the tile. 3. the small break in the glaze will allow you to slowly (no percussion) use a quality carbide masonry bit to make the hole. The bit has a tendenct to "walk away, and off of" your mark without using step 2. hopefully you are not partially on a grout line with your target hole. Good luck!