Dan, as you probably already know, make sure you have plenty of forward speed and altitude.
DONT, pull back too sharply on the cyclic. Pull back smoothly, and you won't lose much headspeed. As you start nearing the top of the loop, pull back to center stick, and as you start exiting the loop, slowly add collective. Also, a good HH Gyro will help keep your orientation, throughout the manuever. Keep practicing and you'll become more comfortable, with it.
REQUIREMENTS FOR HELI STUNT & 3D: 20% skill, 80% nerve
Mike