Practice it from a curb, try to  get both wheels to contact the ground together. A very small, flat  hop (both wheels in the air together) is all you need.

Where the drop-off has  to be taken slowly, on a steep trail for instance, push the  handlebar in front of you as the wheel goes over the edge. The idea  is to get the wheel to go over before you do. Don't grab the front  brake but instead feather both brakes to avoid locking the wheels and sliding. after the drop move forward on the bike as soon as the steepness of the trail allows. The aim is to keep your weight centered over the  balance point of the bike. If you stay way back on the bike, there isn't enough weight on the front tyre to steer  with. This means you'll be likely to crash the next time you try to turn.

The first graphic  shows that the balance point of the bike stays directly between where the tyres touch the trail. The centre of gravity of the rider  is already behind the saddle and will need to go even further back  for the few moments the bike tilts over the drop off.

The next two show how  as the trail gets steeper, the rider must hang further off the back.  Also there's less margin for error as the balance region gets  smaller the steeper the trail.