I have my two more powerful powerheads in each top back corner. The one on the left side is aimed toward the right front corner. The right one is aimed roughly at the center front of the tank. As I don't have a wavemaker this makes for a somewhat variable flow. My two smaller powerhead are in the back bottom, one horizontal, the other vertical to eliminate dead spots.
(this is a 40 gallon (3' by 17" hi by 15.5" wide)

andrew