Each week students choose a book from a variety of supplemental piano material organized by level in my borrowing library. Boxes are marked A-E in order of level. Sight reading is always assigned a level beneath the student's ability- sightreading is not supposed to be difficult or worked on- its just to be sightread.
Once they choose a book they want from the two boxes of their sight reading ability, they take it home to play through it once and only once. I remind my students that they are not allowed to "practice" the sight reading assignment! They really get a kick out of this because who has ever heard of a piano teacher that says "DON'T PRACTICE!"? They are expected to play through the ENTIRE book that week- without working out any particular pieces --unless of course, they have a favorite.
When they come to their next lesson, I choose one or two random pieces and the student chooses another. If they can play the pieces by counting, observing dynamics, tempo, and articulation WITHOUT my spoon feeding it to them or reminding them throughout the piece, they earn credit for that book.
Once they have earned credit for 25 books, they proudly and happily earn this: