Whitewater entry technique depends on how deep it is. Shallow entries (the most common) use a modified belly flop. Protect your face with crossed hands, arch your to expose your mid-drift where the PFD is located, and bend your knees to keep the feet in the air. Jump out horizontally. Here is a good video on the technique: Shallow water entry. Wherever possible use the shallow water entry technique as it is the safest. If you need to enter water you are certain is deep and have to leap off a high bank, use the same technique used when dropping off a water fall - the tucked cannon ball. This will limit how far you drop below the surface and protect your head and back.
An even safer method that is often available is wading until it is swimming depth and then simply push off the bottom into your swim stroke. As with all entries, scout carefully and set your ferry angle on entry.