this brings me back to just use a little tape at the connector.

I use the 3M blue painters tape so it doesn't leave any glue residue if I ever(and I never!) need to undo it,every plane in my hangar has tape on the servo leads for the ailerons,and several have flaps, it costs less than 2/10 of a cent for the amount needed to hold the connector together and thats it.
as for exersizing the connector sorry no.
its a good way to wear out the connector,plug the extension into the servo lead,tape it,shrink wrap it,tie it, glue it,there all going to do the same thing prevent the extension from coming loose in the wing/fuse,and fly it.