Work them out 5-6 days a week, spread the 2 rest days evenly

Sit ups only get certain muscles. Change it up. Go elbow to opposite knee. 

Start to use weight doing sit ups. Every week go up
Example: I use a 25 weight i hold on my chest. 1st week i do 2x20. The next week I would grab a 30 weight 2x20. Work your way up.

Also, once a week, have a day of a LOT of reps and low weight.

And last, 2 sets of Pushups/Hold
DO as many push ups as you can
Then immediately after, get in pushup position but instead of being on ur palms, go on your arms. Hold that position for 60 seconds. YOu should maintain a striahgt back for the whole 60 seconds. It will burn, just fight it.

If you do this, i guarantee 1-2 months you'll see good results.(spring break)

If it starts to get easy, it means your not pushing yourself.