Practice Techniques for Sit Ups, done by David, Clarice and Yun Jing (and his partner)
To improve on sit ups, you'll need to have high endurance and strength so that you can get as many sit ups in a minute. You can train your abdominal muscles by doing the following exercises:
The plank trains most of the core muscles and some lower back to improve posture. It will help with abdominal endurance.
To do the Plank, you'll have to hold your body in a straight line from head to heels. Keep your elbows beneath your shoulders. If you want to make the exercise more difficult, you may put your feet closer together, the closer they are, the more difficult the exercise would be.
The crunch trains the upper abs which would help in the movement of your sit ups.
It is a simple move to train the abs. Start with your body flat on the floor with your knees bent at a 90 degree angle with your feet on the floor. put your fingers/hands at your temple (do not pull your head up so that you can train your abdominals better) lift your shoulders and upper back towards your knees. Make sure your lower back is still in contact with the floor. Keep the pace slow for more tension on your abdominal muscles.
3.) Leg raises
The leg raise exercise helps to strengthen the lower abdominals. (so you'd train your entire core together with the crunches).
Starting from a lying down position, raise your legs until they're perpendicular to the ground and slowly lower them down to the starting position. The slow and controlled movements would help to strengthen the lower abdominals.
Sets, Reps and Schedule
You should try to give yourself a goal on how many sit ups you would like to do, and then break it down into several sets and reps to train, for example 60 sit ups would be broken down into 15 reps of 4 sets. and then slowly trained up. Also you could have a fixed schedule on when you would train.