One of the things that I think helped our guys was in some of our older leagues, you got bonus league points for running different commanders and ships. So sure, you can run Dodonna Liberty for 6 weeks straight, but the guy who plays several different commanders is both going to win the League and get fancier swag than you. And he gets bragging rights, which is worth almost as much as swag. Same with ships and squadrons, too.
Theoretically, also, run a Corellian Conflict game with a few specific commanders they can choose from.