A drop-off is a step down in the trail. The easiest drop-off is one that can be ridden at speed.  Speed hop at the lip to prevent the front wheel from dropping. This can result in a very smooth landing and is one of the easiest types of jump to perform. 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.

See Also:
Technical Downhills
Way steep climbing