Was the mattress on a platform bed in the store? A mattress sitting atop a box spring is going to feel different than on a platform bed.
If not, then you might want to try a mattress on the store's platform beds. If they have no platform beds, you can ask them to put it on the floor and then lie on it and test it. (They might have mattresses in plastic in the back of the store - they could also just throw these down on the floor for you to test.)
Too firm or too soft are both probably going to leave you feeling achey. If you try your best and still end up with a lemon, you could try GDen's egg crate foam. My daughter seemed to do OK on the egg crate foam, atop the horrible mattresses at her college.
The following article about mattresses might help. First, she quotes a National Foundation for Spinal Health orthopedic surgeon, but then dismisses his suggestions because she finds out the foundation accepts money from mattress companies. However, his suggestions seem to make sense to me.
http://www.slate.com/id/93956