An effective ab exercise I do at the gym is called "captain's chair". If you Google- Captains chair exercise- you will find this is an excellent workout for your abs. I do this last when I am at the gym as it requires some good lifting! It is a great one for strengthening your back.

You will also find other exercises you can do under 10 best ab exercises (also in the same area). These exercises were part of a research study to determine which ones were most effective for strengthening your abdominal area. Lots of info here!