As an official, my take away has always been...
Do the best that you can in all aspects of officiating (pre/post season and in season), surround yourself with people that have the same goals as you, and control the "controllables".
Pre/Post Season Work includes, but not limited to...
Weight training/control, cardiovascular activity, rules interpretation study, film study, in person "shadowing", seeking out advice from experienced officials (listen intently), officiating other sports with other officials, networking, etc.