It's very unusual for a mattress to sag with normal use, if your mattress is experiencing body indentations that are less than 40mm on a pillow-top mattress or 25mm on a non-pillow-top mattress then these are considered body signatures caused by the comfort layers conforming to your natural body contours and are perfectly normal. Deeper body indentations may require further inspection.
If you are experiencing a bowing in the middle of your mattress ensure that your mattress is correctly supported on a flat even base that is reinforced with a centre rail and centre support leg, this is necessary to ensure complete A.H. Beard guarantee protection.
The slats in your base should be a minimum of 10cm wide and the gaps between no larger than 10cm. If your base support has flexible slats ensure they are convex and correctly supporting your mattress, check periodically that they do not become concave as this can cause sagging and damage your mattress.