Make sure the mattress is properly supported on a bed frame and box spring. Ideally, the box spring should be purchased along with the mattress as a matching set.
Gather all the bedding and sheets from the mattress, and remove them.
Inspect the mattress. If you have a pillow top or feather top mattress, verify that the mattress has this topping on both sides. If it only has a pillow top on one side, skip Step 4.
Lift up the mattress, and flip it upside down. Do this every few months.
Lift up the mattress, and move it so that the head of the bed section is now at the foot of the bed. Repeat this as needed every few months. This works well for mattresses with pillow or feather toppings only on side side.