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.
Step 2: How to Choose a Mattress
Your goal when you arrive at the store is to quickly determine which two or three mattress sets are in the value "sweet spot". Each manufacturer makes a "standard" line of mattresses and a "premium" line. In the "standard" line, all manufacturers use an inexpensive hourglass-shaped Bonnell coil. Queen sized mattresses of this quality go from $299 and up, but usually no more than $599.
Around the $599 price point you enter into the world of "premium" mattresses with specialized coils offering more comfort and durability. Your ultimate goal is to find an entry level "premium" mattress set. At $599-$699 you can get a queen mattress set that feels great, with guts every bit as firm and durable as a $3000+ mattress.
At each store you visit, don't lay on more than three mattresses. If you do, you'll have quickly lost track of how they each feel and end up confused. I've seen it hundreds of times. Also, make sure when you do lay down on them that you lay exactly like you lay on your mattress at home. People have a tendency to hang their feet off the side and lay in very unnatural positions when in the store. Grab a pillow, lay on your side and put your dirty shoes on the bed. It's OK.
Remember that firmness refers to support and hard or soft refers to the feel. You can have a firm mattress that's either hard or soft. All "premium" coil systems are adequately firm. Your main focus should be determining which feel you like out of the two or three mattresses you're going to try at that store. You will then compare your favorite mattress there to the entry-level premium mattress set at the next store in that same price range and so-on until you find a mattress that suits you.
Step 3: Seal the Deal
Once you've narrowed down a mattress you like, make it easy and give the salesperson an excuse to drop the price. A great strategy I've used multiple times is to decide on two mattresses you like, one more expensive than the other, then tell the salesperson you prefer the more expensive one, but didn't want to spend that much. "If I could get that mattress set for the price of this one, I'd take it."
One way or another, they are likely to offer you a deal before you leave. Either way, repeat steps one and two at another store to see what they have on offer in your price range. Within two or three stores you'll have found a mattress set that feels great and isn't too expensive. You'll probably end up with an extra discount since you'll be looking for it.
It's worth noting that most mattress stores are in eternal "sale" mode. Not surprisingly, the "sale" price is typically the "normal" price. You'll notice very soon that no matter what the "sale" price is, mysteriously you'll always find an entry level premium queen mattress set for $599 in a hard feel and $699 for a pillow-top. Manufacturers weave unique covers and rename beds for each chain of stores so you can't comparison-shop, but with this guide you'll know what quality to expect at what price.
In closing, while I would not buy a used mattress off craigslist because they have cooties, it's a great place to get a generic metal bed-frame for cheap. New you'll pay $40 for a queen sized.
Good luck, and don't forget to ask for a deal!