Marlee,
I'm not sure about sharing lease language, because it's technically not ours (purchased lease), but we do successfully use the strategy you described.
In most cases we make a profit off of early terminations because the tenant forfeits the security deposit and pays the equivalent of another month's rent. That almost always more than covers our make ready expenses, including vacancy. The expenses are usually less than the income from the forfeiture and administrative fee.Then we quickly fill the apartment with a new tenant at a slightly higher rate, and everyone's happy!
In our state (Virginia) we have to be careful with the exact language, so that it cannot be construed as if we are trying to generate double rent off of the apartment. We are simply trying to cover the cost of make-ready, re-marketing, file changes, etc.
Matt