About a year ago we purchased and assembled an IKEA sectional sofa for our living space. A few months later, we notice one of the springs under one of the cushions had sprung loose. And another. By nearly a year later, they all had (except one).
IKEA offers a ten year warranty. But that requires a receipt. People keep furniture receipts? And I would have to return that one section of the sofa. So I figured I would fix it myself.
C-clamp, shim, zip tie. Voila. Reassembled. Easier said than done — it requires a clamp with enough spread to grab the spring near the end, and enough travel of the screw to pull it back into the plastic clips. Wear safety glasses and gloves in case things go pop. Reassembly looked like below (this is actually a photo from later in this post, but you get the point). The zip-tie is to stop the C-clamp from popping out the open side of the zig zag.
Or so I thought. The next morning, all the springs had popped their plastic clips again. What was going on? Take a look at this photo of the springs (below), taken shortly after reassembly. The 
reattached springs are on the left. The springs that did not pop are on the right. What do you notice? They are not the same size springs. The springs on the left half are slightly shorter when untensioned, and if you look at the zig-zags, the ones on the left have a smaller curvature, and there is one more serpentine zig (or zag?) if you count the length on the left vs the right. All the other sections of my sofa have the springs on the right. In short, whether intentionally (they ran out?) or by accident, IKEA used a different and slightly shorter spring on the left, it over tensioned, enough to eventually bend the plastic clips along the front of the couch (where people land when they sit down) so that they no longer would hold the spring in place when faced with even a small amount of vertical movement. You can see on the left (if you expand the picture) how the plastic clips are over-stretched compared to the clips on the right.
The fix. Upholstery clips. A bag of 25 purchased online. 
And 1″ #6 pan-head screws. I removed the original plastic clips (twisted with pliers, cut with knife). I cut a 3/4″ piece of wood to shorten the stretch distance (see pic above) to what seemed right for the spring, anchored to the frame with wood screws. Setting the height to match the springs on the other side was also important (and also for how the cushion sat on it). Tension with the C-clamp, align the clips, screw. It is important for the screw angle to be perpendicular to the clip or pointing slightly into the bend. If it points slightly away from the bend, it may not pull the clip tight (pulling the metal against itself) — ask me how I know!
The finished product – left half has replaced springs. Note I did not replace the spring on the edge. That one did not spring loose, though the plastic clip was still bent. It would have required removing the piece of wood the leg is attached to in order to use the C-clamp. Since it did not spring loose, I left it along and also used 3 zip ties to attach it to the adjacent spring in case it did spring loose.
I re-assembled the sectional sofa and plopped down on the couch a few times for good measure. It is holding!