I've often thought that would be something useful to do, especially since renters often have such a hard time saving (but often are quite able to pay rent on time) - just collect some portion into an account (the deposit account you likely have anyway) and then when they leave fund it all back to them (minus any repairs).

Looks on people's faces when you say "so we needed to repaint, so your $1,000 deposit has left over ... $3k."