Prize per term for the top student each year or form/house with the highest number of hours? It would be cool if Frog could also award a virtual prize too, like a trophy or exclusive outfit or something for the top students but I don't know if that would be possible.
For staff I push the Frog quizzes, saving them time, I also do a Frog drop in over lunch once per term where staff can come in and ask any questions and one was based around frog play and how to find frog quizzes and how to create their own quizzes. Also exam mode has been so useful lately for us, all KS3 science end of year tests have been done this way. 1 staff member made the quiz from old paper ones, this was then assigned by each teacher to their own classes- it saved them hours in marking, and they plan on using it all again next year so they can see the long term benefit already. They used an assigned site with text activities for part of Y9 to allow for more descriptive answers but they rest was all done through timed exam quizzes in lesson time.
We also have some dummy users for staff so they can set quizzes to it and view it as a student would if they are unsure of anything, this has really helped some staff try things knowing it can't go wrong with a fake student account first.