Removable Tape!
Yes, it deserves an exclamation point!
I don't know who needs to hear this, but Scotch makes removable tape. Basically, envision . . . err . . . regular Scotch tape, but with the tackiness and removability of a Post-it Note. So it's basically harmless to most surfaces.
Admittedly, I wouldn't go slapping it on particularly fragile, thin, or newsprint-y items (don't experiment with your copy of Amazing Fantasy #15), but I've never had a problem with any of the cardboard boxes or game elements I've stuck it on. The tape is available at Amazon, but I've seen it at office supply stores as well.
Where it really shines is that – by applying it – you can then use the new surface for NON-removable stickers. For example, let's say you have a nice cardboard box that you keep your Munchkin cards in. You'd like to indicate that it contains five sets, and you even have a label printed that lists the sets . . . but you don't want to damage the box. Simply put down a layer of removable tape on the box that's a smidge larger than the label, then put the label on the tape. If you want to get really fancy, you can put the removable tape on the label first – sticky side of the tape not touching the sticky part of the label – and then trim away any excess tape. Either way, you have an easily removable label; when you need to swap it, it'll just pull away the removable tape and leave the box intact.
As another usage example, if one has an "I'll see you in Hades" attitude toward permanently defacing legacy-type games, removable tape goes a long way toward letting you revert a set close to its original state. This magical tape has other applications that I'll leave to your creativity. Please feel free to share any clever uses you come up with on the forums.
-- Steven Marsh