Pay attention and figure out which way the grain is running off on your first couple strokes with the draw knife. A lot of time I will stroke down the limb to remove wood on one side, and stroke up the limb on the other to prevent the knife from digging to deep.
If the grain is being especially difficult to work with a draw knife, use a sureform rasp and make long even strokes at a 45* angle to the limb instead.
I've lost a few bows to grain tearing, hopefully these will help new bowyers.