I don't think that leaning back very far is necesary in most situations. If you feel like you need to keep the stroke going past your hips, then proper torso rotation through the stroke will allow you to keep your posture upright and centered rather than leaning back. Think about doing a perfectly executed sweep stroke--you don't lean back as you pull the paddle through the stern quadrant, you rotate around your spine using you abs to power the stroke. This is the same motion you want for a good boof stroke, only in a more vertical plane. As Scott says, though, if you do get back during the boof stroke, you can do mid-air ab crunch to get back forward. Sometimes an air boof stroke will help this happen. Boof often, think about landing with your next stroke ready and that will help focus on being in an agressive forward position. .02
edit: check out American Whitewater - Photo#13024
for a great pic of rotating all the way through the boof stroke and finishing foward.