At the house I work at, we implement many of the suggestions that have been listed in the thread. Any employee that signs up a new league bowler, or one that has been away one or more years gets $5 cash. Sign up a team of 4, thats $20, for almost nothing. Its very easy to talk to people. There are always people who come in and practice, but never do league, because they don't think they're good enough. I always explain the handicap system to them, and let them know no matter how bad they are, there's always someone worse.
We have some leagues that are full season, usually around 34 weeks long. Also, we feature some short season leagues, which are around 12 weeks. We have learn to bowl type leagues, bowl this league get a ball type, and big bucks type leagues.
We keep a database, and for bowlers who aren't returning, we have a sales person call them and ask if they will return, or will not. Sometimes its because they can't smoke inside (most people consider that a plus...), sometimes its too long a drive, can't afford it, or they are deceased. If there is a legit reason, ie a problem with the staff, we take note and strive to improve.
I work on Cosmic Bowling nights, and while on that particular night, I make more than from a league, league bowlers are the backbone of the center. We can rely on them being back, week after week, and they'll pay there dues, practice now and then, order some drinks, maybe get a burger, and turn their kids loose in the arcade.
There are some centers out there that understand the way things work (and we're a corporate, Brunswick, run center)