From fhotd member galen98
http://fhotd64476.yuku.com/topic/17548?page=2
Teach her from the ground first. Have her walk actively beside you, and ask her to halt squarely. (You may have to practise this if she isn't used to it.) Stand at her shoulder, turned to face the rear. Use the butt end of a whip to touch where your leg aide will be, and while restraining her from going forward, ask her to move her HQ over. Ask only for one or two steps at first, and be careful to train both sides. She may be awkward at first, wanting to pivot in the middle rather than on her forelegs, but with practise she'll understand. When she's doing it properly, her forefeet will stay within a very small radius, while she'll have a nice clean cross-over in back.
Under saddle, ride parallel to but about 2 meters away from the rail. Halt, turn her head slightly toward the rail, and ask her to take a step over with her hindend (away from the rail, toward the centre of the ring), reinforcing your leg with a tap if necessary. Again, ask for only a step or two at first, and do both sides. Ideally, you want her forehand to stay put, but if she takes a step forward, its no big deal when you're starting. DON'T let her back up. Even though you're asking her to go sideways, this is still a "forward" movement. Backing is a resistance that can cause future problems.
From fhotd member Megsabelle
http://fhotd64476.yuku.com/topic/17548?page=1
I recently taught my runt how to turn on her forehand. I started on the ground, holding the inside rein and pushing her rear over very gently (so as not to get a big movement). When she resisted , I persisted and "turned up the volume" (increased the pressure) until she shifted her weight away from me, at which point I stopped and pet and praised her. Repeat a few times and I found that I could get her to move her butt away with the volume at a 5, then a 3, then a one. When I rode I did pretty much the same thing, just substituted my leg for my hand. Now she'll spin on her forehand like a pro!