The key is to provide random currents in your tank. Your not looking to blast your corals or rock with a direct jet stream of water. To provide random currents you can take multiple powerheads and position them so they basicly shoot water at eachothers streams. This causes turblance where they two streams meet and provides you with the randomness we all look for.
A good startign placment is to put one on opposte back corners of the tank blowing toward the front middle. Another pair in the back blowing across the back of the tank or blowing toward the middle surface. Each tank is unique in how its aquascapes and subtle changes in powerhead placement or direction may be required for best results.