I've had some success with an "I.O.U." book! If a student does not have his/her homework, I request that they sign their name in my I.O.U. book, which is a 3 ringed binder with spreadsheets in it. I have columns labeled Name, date,assignment, made up? and a 4th column for my initials when (and if!) the assignment is made up. This way, students 'acknowledge' the problem, and there is almost never a complaint "But I GAVE it to you!"