PSA: Don't put Rockerz guards in the dishwasher. The springs will rust! You can buy new springs from retailers.
To clean them, I bought a Lysol scrub brush with long bristles, but I still had to trim some of the sides to make it fit in the channel. Someone suggested using a brush made for cleaning narrow tubes, like straws and turkey basters. I tried it today with my turkey baster cleaning brush and it works, but the brush had very-soft bristles, so it took some elbow grease. If you can find a more-stiff cleaning brush, it works better -- maybe from a dollar store?
ETA: One of the pro shops near me will clean out the channel on your guards with compressed air. Not as great as a scrubbing, but gets all the loose stuff out before it can become embedded in the plastic.