Turn your mattress four times a year to distribute the wear evenly. It should be turned side to side and also top to toe.
How to turn top to toe: "Working with one person on each long side of the bed, grab onto the handles and turn the mattress perpendicular to the box spring. The slide the mattress partially off one side, bringing the end that was at the foot of the bed up, and let the other end fall to the floor so the mattress is standing on one end. Then simply let if fall back on to the bed and turn it back into place, with the end that was the foot now at the head."
Vacuum mattress and box springs every three months with the upholstery attachment each time you do the turning.
Strip your bed whenever you go on vacation to air it out.
Don't sit on the edge of a mattress.
Don't allow your mattress to get wet.
Don't use dry-cleaning fluid of any type to clean your mattress.
Don't place a board between your mattress and your box spring - it can interfere with the supporting function of the box spring.
There was one more Don't that we just didn't have the heart to keep on the list, because it just seemed so party-pooper-ish: Don't let children jump on the bed. Undeniably good advice, but no fun either!