As Jay said, DH has some pretty good advise. Also see this page for a decent overview. We can't stress enough that the formica needs to be rock solid, no air pockets or loose areas at all. Many times you can detect voids by pressing down hard in spots and listening for the cement sticking and then releasing, or tap tap tap to listen for air pockets, etc..
Here's the link (copy and paste into your browser): http://popularmechanics.com/home_improvement/home_improvement/1998/1/tile_counter/index.phtml