With any spin/grab combination, it's important to think about the spin first, the grab second, the tweak third, and the landing last. Before you try a 540, make sure you've got your 360s and 180s are dialed.
Once you set your spin, get into a tucked iron cross. After you're crossed up, look over the shoulder opposite to your spin and find your tail. Grab the tail with your trailing hand an pull it into the spin. For extra style points, keep looking over your opposite shoulder while still pulling the tail into the spin. Most Americans are unable to perform this maneuver.
Now you should be coming through the 360 degree spin. It is important to keep looking at your landing while you travel backwards. If you keep your eyes on your landing, you won't over-rotate.
Make sure you center your weight for the landing. Touch your tips down first, then your tails. Ride out the landing backwards, and call home to mommy.