Paperwork when tenant wants to break lease?

[Jon K.]

A tenant of mine wants to break their lease at the end of next month. I have a signed letter from her stating that fact along with her desired move out date. I am about to list the property now to attempt to find a new tenant. Is there any additional forms or paperwork I should have her fill out to cover myself in the event that I do find a new tenant so that she cannot change in mind and stay in the event that I sign a new tenant?

Also, is there any special language I should put into the lease with the new tenant saying something to the effect that it is contingent on the current tenant moving out when she says she will?

I'm just trying to CMA in the unlikely event she changes her mind at the last minute after I've signed papers with a new tenant.