I'm not a PT stud by any stretch of the imagination but situps are my niche--got 110 on my last PFA, and have gotten as many as 121 in the recent past. Here's the workout I use:
Run 4-5x/week
Lift weights 4-5x/week
Ab work 4x/week:
Day 1: 3x max sets of situps, 90 sec rest between each
Day 2: 3-4 circuits of 50 crunches (1-count), 20 bicycles (4-count), 10-15 leglifts on the Roman Chair (picture is below), 10-15 crunches with at least 100 lbs resistance (less if you can't do at least 10 reps) on ab machine (do situps on a decline bench holding a weight plate across your chest if you don't have access to one); no rest between the exercises, 2-3 min. rest between each circuit
Day 3: 3-4 circuits of 40 crunches (1-count), 25 flutter kicks (4-count), 15 bicycles (4-count), 10 hello dollys (4-count)
Day 4: Same as day 2
Here's a picture of what I call a Roman Chair, there seems to be a bunch of different names for these contraptions but whatever they're called where you are your gym will have one (there's dip bars and usually a pullup bar built in as well):
To do the leglifts you prop yourself up with your back against the back pad, with your arms on the armpads and holding on to the vertical handles so that you are in a standing position with your feet off the ground supported by your arms and back, and lift your legs up to form a 90-degree angle.
As for the "ab machine" you use on days 2 and 4, I couldn't find a good picture of it but it's basically a chair with a pad that goes a cross your chest and handles to the side, and you do crunches with an amount of resistance that you set using a pin-selector. Kinda hard to describe but if you see one you'll know what I'm talking about (maybe someone has a picture of one?). Most gyms have them but again, if yours doesn't, you can substitute decline situps holding a weight plate (heaviest weight you can do 10 reps with) across your chest.
You might not be able to do all the reps at first but keep plugging at it. Once you can do that workout make it harder by increasing the number of reps on the crunches, bicycles, and flutter kicks, doing an extra circuit, or adding more resistance on the ab machine. E-mail, IM, or PM me if you have any questions, and good luck!