If you rarely use your futon, you may not care how comfortable the mattress is -- but if it's part of your daily life, comfort matters. Make that futon more comfortable with a mattress topper or by flipping and rotating the mattress frequently. If sitting on the futon while it's in sofa mode is the source of your discomfort, a thin mattress pad may do the trick.
A mattress topper helps make any mattress -- futon mattresses included -- feel more comfortable if the mattress itself doesn't suit your sleep style. A thick topper such as a nontoxic latex foam version fits above the futon cover, beneath the fitted sheet. A thin topper or pad, such as a pad designed for camping, may fit inside the futon cover, so you can use it even when the futon is in sofa position. Just like mattresses, mattress toppers are designed for different purposes -- some add firmness while others make the futon feel softer; still others conform to your body's shape as you rest.
If your futon mattress used to feel comfy but now feels lumpy or uneven, it probably needs to be rotated or flipped. If you sleep on or sit on the mattress on a daily basis, rotate or flip it at least once a month to help ensure it lasts a long time; this also helps keep it from feeling thin or lumpy in one area and firm and even in another. Rotate the mattress 180 degrees one month, and then flip it over the next month for the most even wear. During warm, dry weather, you may even want to take it outside, hang it over the back of a bench or several chair backs, and beat it a bit to help redistribute the futon's filling. This helps the feel of the mattress stay consistent no matter which part you sit or lie upon.