One of our OCs in ROTC would practice them for the 2 minutes to see how many he could do, then work on them from there. The practice times he'd use to work on using him hip flexors for the first portion of the situps, then go to his abs to grunt out the final few once they started to fail. That takes practice to get the flexors to get strong enough.
Another thing you can try is some simple gymnastic exercises. If you have access to dip bars or something, try holding a tuck sit (support yourself upright with your arms, knees tucked into the chest) for a minute of total time, doesn't matter the number of attempts it takes. Once you can hold about a minute on one go, you can extend one leg, and do the same thing (switch up the legs as you go so you don't create an imbalance). Then go to both legs out.
If you have access to a glute ham developer, do situps on it. They're ridiculously hard, especially with full range of motion.
Hope this helps.