Would you buy a car without test driving it?...
A house without stepping inside it (or at least a model of one or have designed it yourself)?...
I went to a couple mattress stores and literally spent about 8 hours one day sleeping / napping on mattresses, listening to all the different spiels these guys will give you, trying out a gizmo that tests for pressure points, etc., but mainly just taking naps...
After about 30 min on each bed, I could tell when I got up if it was gonna work. Some really soft ones felt GREAT when I lied down, but I was hurtin' when I got up (which I'd do really slowly to pay close attention to stiffness, aches, pains, etc.). Others weren't immediately comfy, but I felt pretty good when I got up...
I boiled it down to a couple and then tested them both out, alternating, until I picked one.
I think most people won't do this b/c you look kind of like an idiot in some folks' eyes, lying there on the bed alone with your eyes closed. But... I'd rather look like a homeless guy for a day than feel like I've been sleeping in a parking lot for years after spending hundreds of even thousands of dollars on a mattress that sucks...
-Also, I can't recall the details, but we had a decent money back guarantee on the mattress, too, which was huge. Unless you're in college, just need it for a few weeks or months, etc., or just can't afford anything else, I'd not even consider one without the guarantee.
-Shit, too, I was hoping I'd like a memory foam mattress b/c I've heard that the toppers (if they're at least a few inches thick) give you the same feel but cost a hell of a lot less...
-Last thing is you might consider a sleep number bed (where you can adjust the firmness), but I don't think I found a place locally where I could test one out. Maybe if you've got a friend with one...
-S