Great question!
I would say some of the most important "keys" to being sucessful at high limits (and at all limits in general) are discipline, patience, and the ability to adapt to your opponent's/the game's style (exploiting their mistakes for max profit).
Also the more you can emotionally detach yourself from the $$$ and the results, the better you will do, and the easier time you'll have coping with the mental aspects of the game.