Here's a sample of the letter that I got from the person with whom I initially stayed in France. This letter is neccessary when you apply for your permis de séjour, and should be accompanied by a photocopy of the passport of the person lodging you, and a photocopy of a proof of domicile of that person, such as a gas bill. Just for luck, bring the original of the ``proof of domicile.''
Note: By casually showing the French bureaucrat your huge folder of forms, with photocopies, you can let them know that you're not to be trifled with. This can expedite matters.
Je, sousigné Clyde Wells, atteste que M. John SMITH est colocataire de mon appartement situé au 30 rue St. Jacques, 3ème étage, et ce depuis le 1 août 1999.