How can I prevent myself from using my upper abs and lower back in a lower ab workout?

Every once and awhile, like in the beginning of my ab workout, I will feel my lower abs working when I get to a lower ab exercise, but then, as I progress into my workout, my upper abs and lower back start kicking in, like their trying to do the work instead. How can I prevent this, or fix this problem!?!?!?!?! Thanks for your help!=)