If you feel like you are rolling towards the center in your mattress or if the mattress sags in the middle, it could be from lacking a proper center support. Most beds have slats but not all have slats with legs to give it enough support to prevent sagging. Some frames don't have a center support, most full size frames don't come with a center support even though they should in my opinion. These frames can be complimented with a center support to prevent against sagging and prolong the life of you mattress and box-spring. 
Most mattress manufacturers will not honor their warranty if they find out that a mattress and box-spring has not been setup with proper center support/s.
All full, queen and king size beds or frames should have proper center supports to prevent sagging. If your mattress/looks like the picture below you need to look at either getting some legs for your slats or adding a new center support that will prevent the sagging. Those slats should be level with the floor and have no bows in them.

If your setup looks like above then getting a center support may not fix the problem with your current mattress and box-springs. The box-spring and the mattress may have been slept on for too long and the bow is permanent in both slats and box-spring, sometimes even the mattress has a bow that won't come back after the center supports issue has been rectified. If the mattress and box-springs are new the problem should be fixed right away to prevent any future problems with warranty and/or sagging mattress set.

The use of a steel center support made specifically for your type of frame or bed is the easiest solution. Feel free to contact us or ask about our center supports if you have a problem with a sagging mattress and box-springs..