A couple of people have mentioned that the negatives are labeled. How/where do you label them?

I'm setting up a database to try to make sense of the whole mess (since it's hopeless, at this point, to even try to sit down and put them in chronological order without some sort of tracking system) and I've been trying to figure out a way to give each frame a unique number.

Short of writing over the frame (on the sleeve, not on the negative) with a permanent marker - which I have not yet resorted to - I haven't found a way to do this. Suggestions for labeling (35mm) frames?

Thanks for any suggestions...