No magic: Take a pair pliers to hold onto the valve stem to keep it from twisting, and then take a socket-wrench onto the cap (which is luckily shaped like a nut). And remember, "rightsee-tightsee, leftee-loosee". I think the reason these BMW caps can get tight is because the cap has an o-ring in it, so air that leaks from the stem builds up inside the cap, which makes it tight.