Around here it's pretty damn hard to find a decent rental that allows pets. She should be more grateful that you're willing to rent to her. The next time her lease comes up for renewal give her the option of including a daily clean-up provision (with a reasonable exception for extreme weather) or jacking her rent up about $_____ (put your own dollar figure in there based on the inconvenience it creates for you).
That way you've provided her with the option, and she agrees to a legally binding arrangement. Nothing personal...just business.