The best way I have found for dealing with power gaming is to watch the loot you hand out, limit custom items and the ability to purchase specific magic items(percentile roll based on the community), stick with the core books. If someone finds something that they want to play it should have DM approval. Sticking with the point buy of 15-20 helps. I have always preferred rolling stats myself, but I have each person roll one stat and use that as the standard array. If I have one person who is a power gamer and another who isn't I will try to help the person with their character who isn't the power gamer.
