Leg Raises

Lay flat with your arms at your sides and legs stretched out next to each other, then raise your legs.

Even if you can’t hold them perfectly rigid, keep your legs as straight as possible, and lift them until they are pointing at the ceiling, or as near as you can get. 

Then lower them back down, being careful to keep your movements measured. The eccentric (Lowering) portion should be at the same pace at which you raised your legs. Lower them until they’re hovering just above the ground, and then raise them again.

As shown in the video, an easier variation is to lift one leg at a time. Keeping one leg grounded will help stabilize your body as you raise the other leg, so you can focus on perfecting your form.