The Fontainebleau area is a unique and fragile environment. It is important to respect a few rules to protect it.
Chalk damages the rock, so use it sparingly. You can instead clean the rock (with a soft brush) to increase friction.
When you're done, don't forget to brush the holds (still with a soft brush) to remove any trace.
Sand and dirt under your shoes contribute to polish the rock, which damages it in the long run.
Don't forget to clean your shoes with a doormat or a rug before you start climbing.