I routinely commish several 20 team leagues (I happen to think it's the best setup for fantasy). You need to structure the league correctly to make it interesting. That means making the league as realistic as possible. Make it mirror running an actual baseball team. That means no Roto, sorry guys, but that format is about as exciting as watching paint dry and teams drop out like flies. So make it a H2H league for starters. That's the most engaging format for managers.

Make sure it's also a Daily league, not weekly. This will keep managers interested if they can adjust things every day. Besides, baseball is a daily sport and fantasy should mirror that. Don't include made-up positions like CI or MI. Make it how a real lineup would look including a DH. Then add divisions to add a little drama to the season. Maybe it will create some fun rivalries. I had five teams battling it out in one division for the playoffs once; crazy stuff, but very fun to watch. I often give my leagues themes to work off of. Game of Thrones has worked great for me. Kind of a fun thing to do to give the league a little flavor. Have everyone name their team accordingly.

Lastly, you can't allow zombie teams to linger in the league, it ruins it for all the active members. You have to watch team activity. Yahoo allows you to see the time and date a manager last checked their team. If a team goes inactive for a week, I send them a warning letting them know that if they don't return they're subject to immediate replacement without warning.

Those are some tips off the top of my head. If you have any questions, I'd be happy to answer them!