With a needle, thread, a few buttons, and 15 minutes, you can convert your couch cushions from sagging, shapeless sacks to sculpted, snugly supports.
Step 1: Remove the Stuffing
Step 2: Thread a Needle
Stick thread through the eye of the needle.
Loop the thread and tie a knot, leaving yourself a foot or two of thread to work with.
Step 3: Sew the Cushion Together With Buttons
If you're unfamiliar with how to sew buttons, there are some good instructables (like this one).
I used four buttons to pinch my cushion together in two places. Depending on the size of your cushion and how firm you want it, you might use more or less.