Doing a lot of situps is just a matter of core strength. Go to the gym and get to working on everything you can in that department.
Medicine ball sit ups - exactly what it sounds like. By being forced to keep your balance on the ball you work a lot of the smaller stabilizer muscles (obliques etc) and will be getting a great core workout.
Supermans - it sounds weird, but to have a strong front you need to have a strong back. Lie flat on your stomach on a mat, and extend your arms and legs away from the ground (picture a skydiver falling through the air). Hold the position for a 3count then release. Do a bunch of those. You should be feeling the burn in your middle/lower back.
Obliques obliques obliques - you have no idea how much having strong obliques will help you once you get down the home stretch of your PRT test. Sideways crunches are a good, easy way to go about it.
Leg raises - will help you work on the strength in your lower abdominals, the ones that are responsible for first pulling you up off the floor during a sit up. Do them laying down flat on your back and hanging from a bar or something if you have access to mix it up.
And of course. Do a lot of regular situps. A LOT.
Hopefully that helps. In the end, nothing beats just willing yourself through as many sit ups as you can every day and eventually watching your score rise.