Nothing improves a good night's sleep like the firmness of the mattress. Although you chose the firmness that you preferred when you purchased the mattress, it may no longer meet your needs. Firmness changes subtly over the life of the mattress and what seemed right at the time may now be too soft. Whether the change is a result of your personal preferences due to medical issues, or a result of changes in the mattress itself, when a mattress becomes too soft for your liking there are a few remedies you can try.
Adjust the temperature in your bedroom to 18.3 degrees C. Some newer mattresses that adjust to your body can be affected by the temperature in the room. Generally, higher temperatures cause the mattress to feel softer and colder temperatures create a firmer mattress. Lowering the temperature may make your bed firmer and solve your problems with a soft mattress.
Add a mattress pad or a mattress topper to the bed. This may create a firmer foundation and ease your discomfort. Look for a high quality topper. Check in the bedding supply department of a store for a quality topper designed to provide firm support.
Place a sheet of plywood beneath the mattress. Always check with the manufacturer's recommendations before altering the mattress and box spring, as this may void any warranty you may have.
Purchase a new box spring for the bed. Often the problem with soft beds is due to inadequate support beneath the mattress, and not the mattress itself. A quality box spring may solve your problems and create firm support.