Does anybody now any abdominal exercises that are easy on the back?
I really want to build on my ab muscles but I can never do it because of back pain by the time I do my 15th rep or so. People told me that I’ve been doing it wrong but I know that that’s not the case because I’ve asked to be corrected on it and the problem still persists. Does anybody know what I can do about this?

Strengthening the lower back and improving posture may help.
http://www.exrx.net/Lists/ExList/BackWt.html#anchor150714