There's no mattress material in the world that can stand up to the full weight of a human being or two sleeping on it for months and years at a time. Even the best mattresses break down and develop contours according to the shape of your body. One way to extend the life of your mattress, and perhaps restore a modicum of comfort in the process, is to flip it over every few months. This delays the contouring process, leaving the sleeping surface flatter and firmer for a longer period of time.
Another option to an uncomfortable bed is to buy a topper to put over your old worn out mattress. A topper is way to get that new-mattress feel at a fraction of the cost. In case you're not familiar with the idea, a topper is just what it sounds like -- sort of a mini-mattress only a few inches thick -- that makes it feel like you're sleeping on a brand new mattress. Of course, it won't last as long as a whole new mattress, but it is a way to make your old bed feel better in a hurry.
If you're sleeping on a mattress and box springs -- or worse, a mattress only -- on the floor, there's a good chance the hard surface is interfering with your sleep. Raising your sleeping surface off the floor and installing it in a frame is one way to make your sleeping hours more comfortable in a hurry. The reason this works is that most floors are quite hard and unforgiving. A frame allows the mattress and box spring to flex more, which is kinder to your back and other aching body parts.
Ultimately, there is only so much that can be done to make an uncomfortable old bed more comfortable. If nothing makse much difference, your only recourse might be to buy a new bed. Yes, it's true that new beds can be expensive, but you don't have to shell out for a deluxe model -- even a cheap new bed is likely to be more comfortable than a cantankerous old one.