This is a strategy I got from Harry Wong (great presenter and author!). It's called a Pink Slip, and students fill it out whenever they do not have their homework. I don't listen to excuses, I just have them fill out the form, and I'll look at it later to see what their excuse is. I keep these on file, and they have come in handy for parent conferences (especially if a student is saying they didn't have time to do their homework due to sports or something or the sort).