I really like Sheaf, but I wouldn't want to use those three pronged forks. We usually go with two. You get a better toss off it.

The trick with sheaf is to bend forward at the waist, look up ABOVE the horizon (not down at the fork) and keep your arms stiff. Swing like a pendulum, swing them in the same plane in front of you, not around to your sides.
Always swing higher on the backstroke than you think, it won't fall off the forks. Swing hard, but NOT behind you on the final swing, bring your hand nearest the bag up to about a 60 degree angle from your shoulder, and HIT THE BRAKES HARD! You need that sudden STOP at the end to get the proper flick off the forks.

The final trick is to have someone watch you and tell you where you are peaking, either in front or behind the bar. On your set up take steps either forward or back as appropriate to peak directly over the bar.