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
Before you head out to shop, know that a "good" queen sized mattress should cost you about $599-$799 in the end. My advice here will help guide you to a mattress set you like in that price range which, for reasons I'll soon explain, is a value "sweet spot". If you end up spending less than that, you run the risk of getting a "less than new" or less durable new mattress for your money.
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.