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.

Something that really helped me get faster in the water was using a kick board and alternating with hand paddles. It helps to really strengthen your stroke/kick.

One thing that helped me was alternating with hand paddles and fins. It really helps you feel what movements will help you propel yourself in the water. Also a big thing people miss is working on reducing your time on the wall during turns.