I agree with Coach Jen, sit ups ought to be outlawed. They place too much stress on the lower back and there really are better exercises you could do. If you're stuck having to do these, well then here are some exercises you can do that will help you do the sit ups.
Bridging (regular bridge, not bridge ups)
Bird dog (in yoga, it's called table)
push ups (press ups)
single arm rows
Basically, if you were to do a full body workout along with some core exercises, that will help increase your lean muscle, which will help you to do sit ups.
The problem with sit ups is that people don't do them properly. They use "momentum" to lift their body up off the floor instead of using their abdominals. I blame challenges like this. they encourage people to do as many as they can in as short a time as possible and what happens ? People compromise form for quantity.
Personally, I would consider being able to do 20 military style push ups in good form to be a better indicator of fitness than being able to do 30 with lousy form in one minute. Good form is paramount when doing an exercise.
If you'd like help with your push ups, I recommend trying the 100 push ups challenge. It will teach you how to do good push ups and gain endurance.
go here hundredpushups.com/