I added a 3" Gel Infused Memory foam topper on top of my waterbed mattress. $125.00.
It also has air holes to help cool you if you might tend to get to warm, as some memory foam complaints mention.

There are many copycat brands out now that are less costly than the Tempurpedic brand.
*Toppers won't help much if the base mattress is worn out or saggy..