Whether it is brand-new or years old, a mattress too soft for your comfort level may make for many restless nights. As long as the mattress is in good shape, there's no need to replace it to achieve the desired level of firmness. Instead, purchase a firm topper, slide a bed board beneath the mattress, or add extra support to the bed frame itself.
Like mattresses themselves, mattress toppers come in all sorts of materials and firmness levels. The thicker the topper, the more effect it has on the overall feel of your bed. For best results, visit a shop that allows you to touch or rest upon several types of toppers to find one that feels right to you and is made from a material you like. If possible, test the topper on a mattress with a firmness level similar to yours, so you'll have a good idea as to how the topper will feel on your own bed.
A sheet of plywood cut to the same size as the mattress offers support for a mattress that has lost some of its firmness. Slide the plywood between the mattress and box spring. While plywood adds a bit of support for even a brand-new mattress, using plywood between the mattress and box spring or foundation may void the mattress warranty. Many manufacturers require that the mattresses they sell are paired with specific box springs or foundations to ensure the mattresses wear evenly. If you'd rather not cut plywood yourself, a pre-made bed board, offered at some specialty shops, serves the same purpose. Bunky boards, made for bunk beds, are covered in material similar to a mattress covering. These add support for a mattress placed upon a platform bed or atop a bed frame without a box spring.
In some cases, changing the feel of your mattress is as simple as changing the temperature in the bedroom. Some types of foam, especially memory foam, are firmest when cool, softest when warm or hot. Avoid placing heating pads or heating blankets on the bed if you have a memory foam mattress, as it may make the bed feel far softer than it would be otherwise. In regular room-temperature conditions, the foam is naturally firm on its own, softening as your body heat warms it up a bit.
The bed frame itself may be making the mattress feel a bit too soft or even as if it sags a bit. Remove the mattress and box spring, if there is one, to inspect the frame. A steel frame should have at least one support bar spanning the width of the frame, with a post between the floor and the board, which acts as a stand or leg for the bar. Queen and king beds may require several such supports because of the the wide bed width. Visit a local bed-frame retailer to purchase support bars suitable for your bed frame.