In the world that I commish, I start doing two things right around the time the regular season ends and the playoffs are about to start:
(1) I start polling the current owners to see who's coming back, who's leaving, and who's not sure. Probably to the point of being a pest to some people who don't respond, but it's my responsibility as commish to know what I'm going to be dealing with at rollover; and
(2) I start a recruiting thread in the Classifieds Forum to start creating a waitlist of owners who may be interested in joining our world. Even if I don't know how many teams or which ones may be available, I'm aggressively recruiting. I'm also bumping the thread at least a couple times a day so that I'm always on the first page, and up near the top.
Since we went private 5 seasons ago, we've been pretty lucky in that we've only had 0, 1, 3, 2 and 2 openings over the past five seasons. But for each of those seasons we've had owners already lined up for our open spots, and we were able to fill and approve within 24 hours of rolling.
If you want to fill your world, the commish needs to be active and aggressive during the recruitment process.