So remember how I said exercises that force you to balance are the key to building core strength? Here is another another. People know this by different names, but I like to call it the "Six inch leg raise" because it reminds me that the lower I go, the more effect this has.
How do you do it? Lie face up. Put your palms under your bum, face down so your palms are facing the mat. Keeping your legs straight, lift your legs off the mat until your feet are about six inches high. Try to keep your head and shoulders down, and hold for about 30 seconds.
Now, in reality, it doesn't have to be six inches for this to work. In fact, if your legs were perpendicular to the floor (90 degrees), it would still work. But the closer to the floor your legs are, the harder it is, and thus, the more work your core is doing. So if you are just trying this for the first time, start high, but continue to push yourself to get lower and lower.
Also, why do I say "...try to keep your head and shoulders down..."? This is because it is not easy, and everyone's body is different. You might find that you can only keep your head down to a certain point. In this case, I would advise you to put something under your head so you don't hurt your neck.
Cheers Eights & Weights!