Light oil works well. Don't use WD40 because it will loosen the cable
but will later gum it up even worse.

Best bet is to disconnect the cable on the far end (at the carb), make sure
the wire is good and straight, and pull it all the way out. Clean off any
rust with fine sandpaper or steel wool until it is pretty smooth, oil it good
and put it back in. Work it a few times, pull it out and clean it. Do this
until it is as smooth and easy as you think it should be.

(If you have dual chokes you'll have to get under the dash and pull both wires
through but what you do to the wires is the same.)
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma