Place your feet under a bed or sofa etc.
Put weight on your chest and do the situps (or get someone to pull you down as you go up).
OR
Use someone to stand normally. Place your head in beween their legs so that your head is touching their ankles. Raise your feet to about their chin in height and then tell them to throw your feet down to the ground hard. Dont let them hit the ground, then pull them back up!
You will feel it in the morning!