Rust is created when humidity or water come in contact with metal. The best thing that you can do to prevent this from happening is to properly store your device in a location that is low in humidity. If you have a gun cabinet, this would be an ideal place.
However, many people install their case trimmer to their workbench, meaning that they can’t simply take it out and put it away. In this case, you need to do two things. First, there are gels and oils that you can purchase that you would place a very light film over the entire product. This acts as a barrier to keep moisture from being able to get on the trimmer. Also, regularly wipe down the trimmer so that humidity does not have a chance to build up. This should be done daily.