For those of us who tie down our aircraft outdoors and in the wind (whether continuously or just during an occasional cross country trip), it's important to lock the control surfaces so they don't "flap" unnecessarily in the wind. At the Sebring Expo earlier this year, Juan's Zodiac was fitted with these effective control stops:
Above, two tennis balls are joined by a bungee rope, and keep the ailerons from flapping.
Since this was a show, a tennis ball with a slice in it served as a visible yet soft guard for the wing tips, to prevent folks walking into them:
Two small pieces of plywood (painted bright red) and tied together with more bungee rope keep the elevator secure at each end of the elevator.
These control locks are easy and inexpensive to make, and easy to install and remove between flights.