S.J.
I notice tummy time helps - that is the best way to exercise those muscles. My son hates it - but I know he needs it! Also, allow plenty of time on the floor playing just generally, even on the back. Call your pediatrician for some more suggestions, or even google should help. Babycenter.com is always helpful with these types of questions.
http://www.babycenter.com/0_developmental-milestones-roll...