I've been putting a label inside, and a sticky note on the front of the book saying "I'm a traveling book, please use my BCID code number inside to log my adventures on bookcrossing.com, (anonymously if you wish), then read and release me so I can travel on."
Sticky labels fall off, so I don't use them. They are fine to use though as an extra, a long as you also have a securely attached label on the cover. And please, a spine label, which may need to be taped on, as many times the spine label doesn't stick well. For those of us who scan the books in second-hand shops (which usually means the spines), looking for BC books, it's the only way we will notice them, by those spine labels. I have found several BC books because of their spine label. But I often wonder how many BC books I saw, but didn't recognise, because there was no spine label. They might still be sitting on the book shelf, even years later. I also put a label with the BCID number, etc, inside, on the last page, under the last paragraph of the story.
Some people also write 'Bookcrossing. com' plus 'BCID:_________' on the page edges of the book. I do sometimes, but not always.