Most important thing to do is to stay on top of it. Don't let it get too gruby in the first place, then it never requires much to get it looking good again.
All good comments above... don't forget to do it when the engine is cold, spray it down lightly, then apply your cleaner, let it set for awhile and spray off. I used to use a pressure washer but I think it's safer not to, so now I just use the garden hose. hth