A common mattress complaint can be that it sags, causing neck or back pain or tossing and turning in bed. Some solutions to a sagging mattress can include using mattress toppers, replacing a worn box spring or damaged slates or buying a new mattress set and bed.
When the mattress is sagging, places to check are the box spring and slats. If the box spring seems worn out, then replacing it may solve the problem and be more cost effective than having to buy the entire mattress set.
Similarly, if the slats are damaged, then replacing them or adding more slats for support may solve the problem. Another idea is to place a piece of plywood that is the same size as the mattress between it and box spring.
If the slats and box spring are fine and the problem is the mattress, then a mattress memory foam or latex topper may give some added support. However, if the bed is older than 8 years, then it may need replacing, as noted by the Consumer Mattress Reports website.