If the black-Sharpie-on-a-box doesn't help you easily differentiate between boxes when you're moving, Worldlabel's free PDF will save you time and energy by letting you just stick on your custom labels, whether for different rooms, fragile items or "this side up" boxes.
Worldlabel provides many blank label templates for CDs, addresses and other popular things. If you're moving to a new place though, you'll find that their new moving labels are incredibly helpful. With a compatible PDF reader (Adobe Reader usually works best), you can fill out the forms directly within the PDF, print them out and stick them on your boxes. It's a heck of a lot easier than trying to remember to write "fragile", "glass", and "do not stack" on every box that needs it. Hit the link to check it out.