Require charter schools to set a level of full enrollment per grade level and maintain it. We hear a lot about the number of kids who can't get into charters, but less about the fact that many charters do not replace withdrawn students past the initial year of the school. It's understandable, because it makes their job a lot easier. It also provides no disincentive to letting kids leave the school. If withdrawing students had to be replaced, it would both maximize access to charters, and encourage schools to meet the needs of the students they've got, rather than risk picking a maladjusted seventh grader out of the hat.
If charter schools can't pull this off, it doesn't say much for their model's capacity to expand beyond boutique status. You can't have a city full of schools that span seven or eight (or 13) years with different starting and ending points and which refuse to accept transfers.