How do you buy a mattress?

My back is telling me it’s time for a new mattress. There are so many types to choose from and the models vary from store to store. More importantly, lying one one for five seconds can’t tell you what will be comfortable. I had a Tempurpedic in Florida but it didn’t do anything special for me. Seem to need a medium soft one but not sure about a pillow top. And then’s there’s pricing; they all always seem to be on sale which seems like a con. Any tips?