You’ve probably seen or tried some type of sit-ups on the glute ham developer (GHD).  Here’s how I’ve taken to using the GHD to develop my abdominals lately:

-Get situated like you normally would in preparation to do sit-ups on the GHD

-Gently recline all the way back to the point that you are nearly or completely upside down from about the waist to the head

-With ZERO MOVEMENT AT THE HIPS, ‘roll’ your body up by tucking your chin towards your chest and contracting ONLY with the abs, starting with the top abs and successively moving the contraction further down 

-Reach and hold maximal contraction with the tension focused near your bellybutton, breathe all of your air out while you continue to hold for 1 to 3 seconds, then slowly reverse the movement

-Repeat until you cannot get good repetitions

Feel free to experiment with different arm positions, different breathing patterns, and different emphasis of one side of the abs versus the other.  Lighter people may want to add some load by holding a weight but I find bodyweight to be adequate.

Note that all movement should come from the abs and there should be no swinging or momentum involved.  To ensure this, avoid any bracing or flexing at the hips or knees beyond the basic tension necessary to keep from falling off of the GHD- which mostly comes from your feet and ankles, anyways.  If you do these right, you probably won’t be able to sit up very high at all or get many repetitions.  You should feel an extremely strong contraction in your abs, particularly the top four abs.

Inverted crunches do a much better job of providing the sort of constant tension and pump that leads to hypertrophy in the abs versus GHD sit-ups.  I do still perform GHD sit-ups, but inverted crunches have become my staple because I feel them exclusively in my abs rather than in my hip flexors.  Remember, it’s a crunch or contraction, not a full sit-up like you might be used to.  Don’t worry about getting a lot of reps or a long range of motion, worry about the intensity of the contraction in your abs on each rep.