Just like any other type of mattress, a futon mattress may require cleaning from time to time. The more you use it, the more it may absorb odors and even sweat. If the futon functions as a couch as well, spilled drinks may be an issue. Keep that futon fresh and clean by vacuuming it frequently and spot-cleaning it as needed.
If your futon mattress houses funky odors, it's time for a freshen-up. Sprinkle baking soda over the mattress while the futon is flat. Wait at least 15 minutes, and then vacuum the mattress using an upholstery-brush attachment. Flip the mattress over and repeat the process. This freshening process also works for the futon cover, if you cannot remove it to wash.
Air It Out
Even though the futon mattress may be bulky and a bit unwieldy, a good dose of fresh air goes a long way to keep the mattress smelling its best. A futon mattress absorbs moisture in the form of sweat and even humidity in the air, so drying it out helps keep it fresh. Take the mattress outdoors, if possible, and rest it over a porch railing or across the backs of several chairs to help it air out on a dry, non-humid day. After an hour or so, flip it over and air out the other side. Cover the mattress with an old fitted sheet to help ensure it stays clean as you air it out. If you can't take the mattress outside, lean it up sideways against a chair or table indoors in a well-ventilated room, turning on fans to increase air circulation.
The sooner you deal with any type of spill or moisture on the mattress, the better. Blot the wet spot up with folded paper towels; avoid rubbing, as this may only spread the liquid around, causing a bigger mess. If the liquid is anything other than water, dab a soft, damp cloth over the spot to help dilute the liquid, and then blot it up some more. Address odor- or stain-causing liquids by rubbing them with a squirt of a mild liquid laundry detergent or dish soap applied to a damp soft cloth. Rub the stain from the outer edges toward the center to avoid spreading the offending substance. Wipe the spot down again with a damp cloth, and blot the area dry with a fresh cloth. Allow the mattress to air dry completely before using it or putting a cover back on it.
Keep It Clean
A little time spent protecting the mattress is well worth the effort, because it will stay clean a lot longer. Purchase a washable futon cover, if yours does not already have one, and keep it on the mattress at all times -- except when you need to wash it. If you use the futon mattress for sleeping, place a regular fitted bed sheet over the futon cover to protect it even more. Futon mattresses are made in standard bed sizes, so a fitted sheet designed for the same mattress size will fit the futon mattress nicely.