I always use my deposit towards their first 2 weeks of care that way I never have to worry about keeping the deposit for later use. I also require 3 weeks notice when someone is leaving. 2 weeks is never enough.
On a side note - it really bugs me when people know they are leaving and leave it to the last minute and blindside me. I actually put it in my contract that I ask for people to communicate with me their future plans so that I can maintain a proper waiting list. I had a mom once pick up her daughter at 5 on Friday (she had paid me for the next 2 weeks that morning) and tell me her daughter was done and not coming back those 2 weeks. I was so schocked. Now 3 weeks is required and I will likely push it to 4 weeks eventually.
This has to be the "look out for myself so others can't screw me mentality" where they feel if they wait till last minute then the daycare provider can't end care before they are ready for it. It sucks for the kids who don't have time for good byes and all that.