To help the upholstery fillings of the bed to distribute evenly, you can try turning over the mattress every one or two months, and for new mattresses, do it more regularly on the first few months. Try not to bounce on the bed or sit on the edge of it.
When it is time for you to replace your old or spoilt mattress, please don't ever think about giving it to anyone of your relatives and friends because it is not hygienic and very likely to cause a health hazard. Ask the store that you purchased your new mattress from if they can arrange for a disposal. Also, it would be wise if you could also purchase a new base for the new mattress because they are designed to work together.