My booty instructor (who also instructs pilates) advises abdominal curls rather than sitting up with a straight back, and only coming up to the point that you would have to either grab your legs to come up further, or your feet come off the floor.
It's kinda hard to describe in writing, I'd make a hash of it if I tried, it's best demonstrated, but here goes...
Make like you're doing a sit up (on the floor, knees bent), hands behind your head and you should not ever see your elbows. Anchor your pelvis to the floor (pretend like there's a weight holding your belly button to the floor) and kind of roll your shoulders then each vertebra up off the floor. The movement should originate with the abdominals, not your back, it's almost like your pulling your rib cage closer and closer to your pelvis with your guts. Then do the same in reverse to go down.
You can make it harder by picking your feet up off the floor (start with them at right angles, then extend them outwards for increasing difficulty), but your lower back should not arch
, keep it anchored to the floor like someone's bolted it down . If you like you can do pulses - come up to that point then contract your abs a teeny bit more a few times at the top so you kind of look like you're bouncing at the top of the "situp".
This one is a killer too - sit with legs bent in front of you like you're at the top of the sit up, have a partner do the same a bit behind you and both lean back to the point that you can hold it without bothering your back, again, no arching the back (adjust so you're just about touching each other's backs). Now take an object (ball, cardboard box, boxing pads, cushion, whatever) and pass it to each other side to side holding the object with both hands (so you're twisting side to side). Gets the oblique muscles, but be VERY cautious of your back.
Then there's planks - off knees or toes and on your elbows or hands (like you're going to do a pushup) make your body like a straight board (bum not up or down) and hold it for as long as you can. If your bum's right you'll feel this in your abs, if your bum's up your arms will hurt, if it's too far down your back will hurt. Make sure your shoulders are away from your ears so you don't get a stiff neck/shoulders and your hands should be under your shoulders. Make it harder by picking one arm or leg off the ground, or when you get really good, an opposite arm and leg.
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"