Years ago I saw a cute little homework pass that read "The dog ate my homework" and that brought a smile to my lips. I decided to do something similar for my own homework passes, but in Spanish of course.
To save paper, I fit four of them to a sheet. There is a space at the bottom where it says "firma" and this is where you sign the homework passes to validate them. You don't want some entrepreneurial student of yours to photocopy them, of course. For increased security, you can copy them on special paper.
There are directions on the pass for use, students write their name on the pass and the name of the homework they want to skip. Then they staple it to the homework and turn it in with the rest of the stack. Then, while you are grading, you will come across the paper and you can take care of it right then and there. That is the theory anyway, your mileage may vary. :-)
I try to save these for very special occasions - partly because I want them to be a big deal, but mostly because I try only to give homework that I feel is important and, because of that, I really don't want students to skip it.
Use them well, amigos!