The strength-and weakness-of futons is their adaptability. Many wooden futon frames can be folded into a couch, or laid out flat as a bed. The trouble is that a futon that doubles as a couch or a bed usually spends most of its time in one configuration. If you use it as a bed for years at a time, you're likely putting stress on the wooden slats that they weren't intended to endure. Follow these steps to find out how to trim up the sagging frame.
Figure out the extent of the damage. If your futon frame has wooden slats, check to see if any of the slats have worked loose from the main beams running up and down the length of the frame. If they have, they'll require some repositioning. By the same token, if your futon has been feeling saggy lately, check to see if any of the slats have broken in half, or are on the verge of doing so.
Remove the futon completely from the frame. This is the tricky part. Futon mattresses are notoriously heavy and awkward; so, it'll be tempting to uncover just a corner of the bed frame in need of repair, rather than pull the entire mattress off and stash it somewhere temporarily. If you don't completely remove the mattress, though, some portions of the frame will be bent out of shape while you attempt to fix others.
Gather the appropriate tools. If any nuts or screws are working loose from the frame, you'll need a screwdriver or hammer to put them back in (if the screws are worn away, you may need to find new ones). Broken slats can be repaired with wood glue (read the instructions on the tube carefully). Slats that have worked themselves loose can be tied in place with liberal applications of duct tape.
Slide the futon back on. After you're sure you've put on enough duct tape (or that the wood glue has dried), slide the futon mattress back onto the frame. This is the second tricky part. If you stress the frame too much while trying to replace the mattress, you may cause more damage than existed in the first place, or undo your repairs.
If all else fails, sleep on the floor. After they reach a certain age, futon frames can be damaged beyond repair. Unless you want to repeat the above steps every week for the rest of your life, consider dismantling the frame entirely, throwing it out, and putting your futon mattress directly onto the floor. It'll feel funny at first, but you'll get used to the new arrangement after a few days. On the other hand, if you don't want to sleep on the floor, you can simply buy yourself a new bed frame.