Wire coat hangers! These things work great, and they are typically cheap (or free) in most cases.
To cut the coat hanger, use the 'cutter' part on your pliers!
Don't use Chad's wire cutters to cut coat hanger wire.
Running a wire loop around the wing spar is another great way to create a hanger mount.
Running the wire through solid parts of your RC plane is the best practice to ensure your plane doesn't fall or warm after a long time hanging.
Don't use rubber bands! After a few weeks, the rubber will dry rot and your plane will be on the floor.