Butter is the only binding agent in a graham cracker crust, so if you have had problems with your crust sticking together, it's most likely the result of the butter to graham cracker crumbs ratio being off. If you decide you want a thicker crust, you can't just add more crumbs (say 2 cups) without simultaneously increasing the butter. At the same time, you don't want to make your crumbs soggy with melted butter. Another important part of getting the graham cracker crust to stick together is to apply some pressure to it. I always use a glass to do this, tamping and packing the crumbs together, but a flat-bottomed measuring cup works perfectly well also. The final step is to set the crust either by baking or chilling. Baking the crust will melt the sugar and help bind the crust together. Chilling with cause the butter to firm up again and help the crust hold together.