Having your general command in battles where the odds are even or against him is the best way to gain command stars. Just tagging along as a second general in the stack won't do any good. It is also good to give him rest between battles if he doesn't immediately gain a star so he can think about what he did right and what he did wrong so he'll do better next time. The stars are nice to look at, but the morale traits are more useful on the battlefield. Putting the general in the thick of battle is something that can help out there, with traits like Brave resulting.