Before going off to school I served a few years at a national mattress store chain. Ever since that experience, family and friends have counted on me to help them get a durable mattress set they like for a reasonable price. I'll give you the same advice I give them so you can get a comfortable mattress without paying more than necessary.
Step 1: Before you head to any stores
There are three types of conditions the mattress you're looking at will come in:
1. New, never opened bag.
2. Opened bag, many times a signal that it was returned on a 60 night comfort guarantee.
3. Floor model, which could be the truth or it was a return at some point before they used it as a floor model.
You want to insist on picking-up or having them deliver a mattress in a new, never-opened bag.
Pick up to three local stores to visit, preferably stores that only carry mattresses. Beware of mom-n-pop stores selling name brand mattresses without the name brand logos in the front windows facing the street. Authorized dealers selling new mattresses can use the logos. If they offer you a great deal on a Sealy but it isn't in a sealed bag and they don't have any more like that one, odds are they bought a truckload of comfort guarantee returns that have been slept on by strangers already.