It really depends on a few different factors. What shape is your tank? Cube, rectangle, corner, etc. What kind of corals are you looking to stock? If you're going to grow lots of stags, I'd suggest keeping the rocks lower in the tank (or at least a section of them) to allow the stags to grow tall.
You can go with more unique shapes by drilling and using pvc, zip ties, epoxy and super glue.
How many sides can you view your tank from? Remember that changes in vertical height are harder to see when you're straight on than viewing the elevation changes from the side (hopefully that makes sense).
No matter what shapes you end up deciding to use, make sure you keep as much rock as possible off the sand. And the rocks that rest on the sand should actually be resting on the bottom of the tank with the sand going around the base of the rocks. This will keep things from settling if you add an digging fish or inverts later on. Once you have decided on how you want everything placed, then start worrying about adjusting your power heads/return so that you maximize flow through and around the rocks.
