1 - RUBBING
Old and well known method.

A few tips : use a semi-hard pencil (reference 2 H seems the best), and slightly glossy paper, not too thin (pages from a pocket-diary are usually good). Avoid absolutely: newspaper, tracing paper, toilet paper. Rub regularly, holding the pencil at an angle of about 45°, and always make two rubbings, as different details may appear.
Be careful not to rub too strongly if the mark is worn.

2 - SMOKE BLACK
This method gives spectacular and immediately usable results, often better than a rubbing. Photo may be preferred in the case of worn marks. It is harmless for the pieces.

EQUIPMENT : candle, transparent adhesive tape, hard glossy paper.

METHOD : light the candle and let it move under the mark so that it is covered with smoke black (the same result can be obtained with a lighter, it is quite longer and not always harmless for the fingers!). Stick a piece of tape on the blackened mark (be careful not to move the tape), press slightly with top of finger and transfer tape to paper.
In case of large marks, use two pieces of tape side by side.

3 - PHOTOGRAPHY

Undoubtedly the best method, but requiring some technical knowledge, as well as specific equipment: spotlights, camera stand, macro-lens. Enlargements may reveal unspotted details.
Recently on the market, digital cameras can make as good pictures, immediately transferable on a computer for improving or enlarging.

Photo traditionnelle

Photo numérique

4 – SCANNER