The best way to keep good axles in shape is to tow the trailer.
Many will recommend taking the weight off of the axles when the trailer is stored. I'm not convinced this method helps. The idea being, whether they are pushed up by weight or hanging down for the lack of it. They still freeze in place because of lack of exercise.
When a trailer sits for long periods of time the rubber hardens and loses it's elasticity. By using the trailer you extend the life of the rubber torsion springs. While they don't last forever, exercise does help. Much in the same way it works for you when you exercise.
Knowledge: "A gift to be shared. A treasure to receive."