If your mattress is sagging or you simply like a very firm mattress, there are ways to achieve this temporarily before buying a new mattress set. All you need is a bit of ingenuity and a small amount of cash to make your existing mattress extra firm until you're ready to investment in a new mattress and box spring.
Place your mattress directly on the platform of your bed if it has a solid base, or directly on the floor without the box spring. This is especially helpful if your mattress is still good but your box spring is worn and sagging.
Insert a piece of plywood between your mattress and box spring. The plywood should be at least 3/8-inch thick. Cut the plywood slightly smaller than the box springs so that the plywood corners don't stick out. Use two pieces of plywood for a queen-size or a king-size bed.
Place your mattress on wooden or plastic pallets. This will achieve firmness while raising the bed off the floor, which will make it easier to get in and out of bed.
Use a folding bed board between your mattress and box spring. Commercial bed boards are covered, which decreases the likelihood of damaging your mattress. Purchase one for a twin-size bed or two for a queen-size or larger bed.