Get a new box spring. The box spring is the support for your mattress, and when the mattress gets worn out, a new box spring may be able to help keep it level.
Use the upholstery attachment of your vacuum cleaner to fluff up pillow top mattresses. Run the vacuum nozzle directly over the top of the mattress. This will pull up the fabric and make the cushioned portion more comfortable.
Rotate your mattress. Every few months, turn your mattress so the top end is facing the bottom.
Flip your mattress if it's not a pillow top. Every six months or so, flip the mattress completely over so you're sleeping on the opposite side.
Add a mattress pad. This is an excellent way to make a worn mattress more comfortable. Mattress pads can be up to five inches thick and are available for all mattress sizes.