Does anyone have tips on shopping for a new mattress?

The whole idea gives me a little stress. I just don't think I can get any kind of idea what a mattress is going to feel like long term, when all I can do is lie on one for 15 minutes in a big showroom, fully clothed with no sheets, blanket, mattress cover, etc. So how do I pick?

My wife and I, for the past five years, have been sleeping on a hand me down king mattress that must be about 25 years old now. The center of the bed is about two inches higher than the divets on the left and right. No amount of flipping will do this mattress any good Frankly, I don't remember ever regularly sleeping on a new mattress, so I'm very lost for what to look for.

What I do know is my utmost concern. My wife might have other criteria, but I'm looking for the mattress that keeps me the coolest! I run very hot, and that's my biggest problem when I'm sleeping. I went into a mattress store recently and shared this issue with the saleswoman. She said that there was a brand new line of mattresses coming out with these "eGell" discs in it that keep you cool. This was for a memory foam mattress (I don't know what brand).

Up until talking to her, I hadn't even thought about memory foam. I've never slept on one before. I've never even spent more than a couple minutes sitting on one.

This leads me back to my main concern: how on earth do I decide between mattresses - something I'm going to spend a third of my time lying on - when I have so little time with them? Most places seem to charge you a nice fee if you return it...

I wish there were something like a mattress hotel, where you could spend one night lying on a mattress of your choice...

Do you guys have any tips?