Buying a new mattress brings with it issues of care, the time it takes to break it in for maximum comfort and even wear. King Size mattresses are the largest mattress size available and will require adequate support from the box springs or platform, as well as from the bed frame itself. It's essential for the longevity of the mattress that a frame designed for King Mattresses be used when using a box spring set. Without this support the mattress will be prone to sagging and it can greatly reduce the comfort of the mattress in a short period of time.
Flip the mattress. Even when the manufacturer states that this isn't required, to ensure even wear, flip the mattress from head to foot every other month. This will help prevent the ridge that forms because the padding has been compressed during normal use.
Turn the mattress over. Besides flipping it from head to foot, turn the mattress over. This should be done during the months when the mattress isn't flipped from head to foot. This will allow the padding on the other side of the mattress to decompress.
Nap in the middle. Taking naps in the middle of the bed, or sleeping there when space permits, can also help prevent a middle of the bed ridge.