Perhaps that's too forward. Try some humor. How about a statement on a napkin ...perhaps some test runs on others to see how it goes...but just because it's works for some, may not guarantee it'll work on her. Just my thoughts and I'll move forward as to what I would do! Just don't get fired bro!!!
In efforts to be more green, we ask that you rinse out your cup and return it...the person that used that same cup before you, well, they FORGOT!!! Any questions or comments, text me (insert digits) Thank you and have an AWESOME day!
Don't forget the happy face after the FORGOT in caps...either way, she'd have to be a complete doofus to take it seriously!!! I left out the name purposely to arouse interest.