Calligraphy pen, black India ink and clear nail polish over it is best. And write as small as possible on the worst side (like the break, if broken). Many don't like to do it, but it's worth it. Simplest way is to just give it a number and keep a catalog/notebook (keeps down on the writing, after all you don't want to write a "book" on your piece!). You can list the site/location, date, and any other kind of notes. It's a good thing to do, because years down the road you'll still know when and where each piece was found. No one's going to be around forever, so it's nice to pass that info on to future generations (especially if you're lucky enough to have a kid that's just as interested as you are!).