Purchase a pillowtop mattress pad. Pillowtops can be found in the same sizes as normal mattresses (twin, long twin, full, queen, king, and California king), which makes it possible for you to get a perfect match for your bed, and they can also be found with differing levels of softness and depth. If you are looking for a more supportive pillowtop, choose one made out of memory foam or regular latex foam. If you prefer softness over firmness, choose one made out of feather down or wool. Depending on your preferences and/or allergies, you can also find pillowtops made with and/or without specific materials. Look in the materials listing when you're searching for a pillowtop to find one made out of wool or foam, or find one with a "Hypoallergenic" label on it.
Remove everything from on top of your existing mattress, including pillows, blankets, and all sheets.
Place your new pillowtop mattress pad on top of your bare mattress, making sure that it is right side up. Position it evenly on all four sides.
Replace your fitted sheet, making sure that it fits nicely over both the pillowtop mattress pad and mattress. If it seems too tight or if it keeps coming off, you may need a deeper fitted sheet.
Place your blankets, pillows, and other sheets back on the bed.