Generally, I follow a strength training program that involves many series of low reps, done slowly. When I do my situps, I will do them in groupings of 15, slowly performed, with a brief half second pause in between each, in order to kill momentum. The neck and head is kept straight in order to prevent any attempt at pulling myself up with neck instead of my abs.
Following this, I begin to alternate pushup/situp routines. Here is the ab routine I follow:
1. Regular situps
2. Alternate Left/ Right
3. Elevated situps - Using a bench from bench press, with feet locked in (usually followed by elevated pushups)
4. Keeping feet/ butt elevated, reaching up with both hands as if pulling a rope down (double reps)
5. Knees bent - pull both knees up and in toward chest as if I had a ball in between them. Back remains straight.
Once this is completed, repeat until unable to continue. The advantage is that by doing them slowly and correctly, you will gain a great deal more from each technique. By alternating low reps between pushups and situps, you will find that throughout an hour or so, that you will have done about 5 times as many pushups/ situps as you were normally capable, with better technique on each movement.