One guideline for me is to try not to stack any rocks against the back glass. A lot of people pile their rocks against the back wall to give their fish more swimming space out front, but forget that in doing this, they are more than likely creating dead spots. Other than that, I try to create a mental picture in my head of what I want the rock work to look like and then try and replicate it.
+1 on not placing them against the side walls. Bad for flow, and glass cleaning. Also need to create holes for your fish to hide in. Must be pleasing to your eye. Need to have many places to mount your frags. Some high, some med, some low. depending on light needs. And it should look natural.
I drilled my rock and inserted acrylic rod to connect them. Some were drilled and epoxied together others just stack up on the rod. The rocks on the bottom of my aquascape site about 1" above the sand in most places and have acrylic rod that connects them to 1/2" acrylic plates that site under my sand. Keeps everything in place real nice.
its nice to have the top layer smaller loose rock ( but stable) so when you fasten coral to it you can pull the whole rock if you want to frag or move it around.