Sleeping on a sagging, lumpy mattress doesn't do your back any favors, and a poor night's sleep can interfere with your waking life. When it's time to replace your mattress, invest in a comfortable new one, but don't simply throw the old one away. If you leave it out with the trash, you might find it lingering on the sidewalk at the end of the day -- mattresses are too large for many municipal trash collectors to pick up as part of their daily route. Besides, a mattress that no longer serves you might still be useful to others.
Contact your local thrift store or community charity to see if they take old mattresses. If your mattress is in terrible condition, don't pass it along to others, but if it's still useable, charities and thrift stores might want it. Some organizations are able to collect large items such as mattresses from you; others need you to bring the mattress to them.
Place a "For Sale" ad in your local newspaper or on a classifieds website such as recycler.com or craigslist.org. Used mattresses generally sell quite cheaply on such sites, so don't expect to make much money. However, if you specify in the ad that you are unable to deliver, the responsibility of finding an appropriate vehicle to move the mattress falls to the other party.
Look up mattress recyclers in your area in the phone book or on a database website such as Earth911.org. Recycling mattresses is a specialty industry, so there might not be a facility in your area. If there is one, take your old mattress there, particularly if the mattress is no longer in good enough condition for someone else to use.
Ask the retailer who sells you your new mattress whether he can collect your old mattress. Many retailers can remove used mattresses when they deliver the new ones, often for free.
Call your local municipal trash collector and schedule a time for a large-item pickup if all else fails. Meet the trash collector on your curb at the determined time and give him the old mattress for disposal.