1. Fluff it up

Most saggy couch cushions can be fixed with no tools, payments, sweat or frustrations. You'll find a hidden zipper in the upholstery of most sofa cushions. All you have to do is remove the cushions, unzip the upholstery and manually fluff the filling. Cushions are sturdy fabric wrapped tightly around loose, movable filling. When you put weight on a section of it, the filling moves and condenses. Some can be pushed out to the sides, and wherever you sit directly becomes compressed. 

Here's how to fix saggy cushions:

1. Remove the couch cushion that needs fixing.
2. Find the hidden zippers and unzip them. 
3. Check for secondary zippers (usually there's a zipper for the upholstery, and a second for the casing around the inner material. Should this be the case, unzip that zipper as well. 
4. Reach your arm in to where the sagging is an issue - this is usually right around the center where people generally sit. 
5. Use your hands to fluff up the inner material, pushing the cushion back into shape and making sure to keep the corners and edges filled in. Any material that appears flat and condensed should be separated and fluffed. 
6. Put everything back and take a look! Oftentimes that's all it takes to make your couch look as good as new. 

2. Add more filling

This is especially true for older, well-loved couches, where the sofa cushions might need a little extra help to regain their former shape. In which case, don't be afraid to re-fill them. There are a number of easy, cheap ways you can achieve this. You can usually buy more of the same kind of stuffing. If you have poly-fil cushions, buy extra poly-fil. And if you have down cushions, buy extra down.

You don't generally need to go that far or be that exact, though. You can use any kind of stuffing, really, for the extra volume, since it's just adding a bit of extra fluff, it doesn't need to be the most-high quality stuffing. Another easy option to add volume and fluff to your saggy couch cushions in using foam egg cartons. Just cut one up and slide the pieces into your couch. They provide both structure and volume and will maintain comfort and look for longer than other forms of filling. 

3. Slip some plywood underneath

You can buy specially-marketed 'furniture fixers' or 'couch supports' to fill your couch, though it is a lot more expensive, and will probably be no easier than making one yourself. Alternatively, you can opt for a cheap piece of plywood. To fix your sagging couch cushions using plywood, you need to first remove the cushions.

Measure the area of the couch over which those cushions go and cut a piece of plywood to that size. Slide it in, replace the cushions and sit down. While it may seem odd to put hardwood in your comfortable furniture, it's important for your cushions to have support underneath all that comfy fluff. A simple piece of plywood can keep your cushions from sagging. 

4. If you don't have zippers, don't fret

Some couches don't have the hidden zippers. So, if you find that you are trying to solve your saggy-cushion issue but are discouraged by your inability to get in there without a zipper, don't worry. Your first option would be to cut a piece of plywood to fit into your couch and slip it underneath the cushions. Alternatively, non-zippered couch cushions have seams. Look underneath the cushion and find a sewn channel. It's recommended that you use an actual seam ripper.