I just replaced the couch cushions recently, and was about to throw the old ones out. Then I remembered reading this tread! I used a bread knife, and cut 3 slots in a u shape in the side of the foam cushion. Did not remove any foam, just deep cuts, all connected.
Then, a dribble of lube in a condom, which was then put on. To prevent condom loss causing the cushion to become a cum-dumpster, I put a plastic bag over the erection, then pushed it into the foam cut. Not bad, but could not get the depth wanted. Reached deep into the gap, and used fingers to open up the foam a bit deeper.
Thrusted back in, and was surprising pleased with the feel! With the cushion on top of the new one, it was at the right height for some kneeling thrusts. Could even do some ball slapping!
Going to cut some other shapes tonight, an "H" shape and a "V" shape. Again, will make cuts deep into the edge. Will not remove any foam, at least for now.
Something to keep in mind when you see those "free" couches on the curb....