I was wondering if you had any advice or exercises to help my sit-ups?

The most easily correctable things I see guys do is

1) hips are too close to your heels

2) doing negatives on the way down.

That will give you a few more, but overall I would recommend just varying your training if you can. When you are doing ab exercises look up some ab exercises you have never done before and just do a 10 min session of whatever that exercise is and you will see a difference.