I started off trying to be flexible and understanding and found that I was flexing too much. This caused a lot of stress for me. If you accept late homework, you will receive a wave of late work at the end of the quarter. I have tried making the penalties stiffer and stiffer until I actually saw some results. I tried 10% penalty for late homework (little impact), then tried 50% penalty (that made a difference). In some classes I do not accept late homework at all (this way, we can grade the homework together in class). There is complaining (once or twice) and then they realize that they need to be prompt - so 90% of the students are on time with 90% of their homework. I also don't do make-up tests. Period. (Well, if your father is in the hospital, or you are in the hospital/jail, we will talk about it. Other than that, you just get a zero for that test/quiz.) This may sound harsh, but it is working for me. I usually do only one make-up exam all quarter in all classes combined! I tell students that if they are late to the airport (whether a good excuse or bad excuse) the plane will leave without them. They get it. This has reduced some of the end of quarter grading stress that I was experiencing.