Your mattress cushions your body and supports it as you sleep. A lack of support causes you to wake up sore and aching, feeling as if you have not slept at all. Pillowtop mattresses have an extra layer of padding on top that makes the mattress softer. A pillowtop mattress, however, can be made firmer and more supportive if you dislike the softness. The support offered by the mattress has little to do with the layer of padding on top.
Remove the pillowtop mattress and the box spring. Inspect the bed frame. A sagging bed frame causes problems with how supportive and firm the pillowtop mattress can be. Tighten all the screws on the bed frame with a screwdriver, or if there are significant alignment problems, take the frame apart and put it back together.
Remove the bed frame and box spring. Box springs and bed frames are meant to give the mattress support, but if they are old and not doing their job correctly, they can cause the pillowtop mattress above to cave in toward the middle or sag. Lay the mattress directly on the floor and try sleeping on it for a few nights to see how the support improves.
Lay a foam mattress pad on top of the mattress. If you are sharing the bed with someone who prefers the softness, use a foam mattress pad that is narrow enough to only cover your area of the bed. Place the mattress pad between the sheet and the mattress, laying pieces of shelf liner between the mattress and the pad to keep it from slipping. Make the bed as you normally would. Use safety pins to attach the foam pad to the raised edges found on a pillowtop mattress. This prevents the pad from slipping away.
Replace the pillowtop mattress. A pillowtop mattress is designed to be soft but it should still be supportive. If your pillowtop mattress is not giving you the support you need, choose another that has a higher number of springs and a lower spring gauge. Both result in a firmer mattress.