How to keep graham cracker crust from falling apart
It's unrealistic to expect a graham cracker crust to slice perfectly. However, it shouldn't fall apart completely. The problem could be in the ingredient amounts, the baking time, or in not pressing it effectively. Check this out

http://www.makeitlikeaman.com/2014/01/11/graham-cracker-crust/

and see if it helps. It has a good method for pressing.