In France, being able to prove where you live is extremely important. Everyone wants this proof, called Justificatif de Domicile, from the préfecture to CPAM, to your bank, and so on. It is as important (if not more important) than your ID! It is usually a utility bill or rent receipt that is in your name and shows your address (and just so you know, a cell phone bill won't work, but a fixed line bill will).
So, what do you do if you don't have any of these? Say you are staying with friends or relatives, or renting a place that includes utilities; there is a solution! You have your host write a letter of attestation d'hébergement* (stating that you live at his address), and provide his ID copy and utility bill. Sounds easy (and it is), but of course, it has to be in French, and it's formal (Ack! By the way, most correspondence in France is formal).
*Here's a handy form to use (note that it's for a man hosting a woman; change the verb forms for a different situation). We got this originally from a French consulate site, but of course, I can't find it now!
[relative's name and address]
À [relative's city, date]
Objet : Attestation d'hébergement
Je soussigné [relative's name], né le [birth date] à [birth city, country] et demeurant au [current address] atteste sur l'honneur que [your name] née le [birth date] à [birth city, country] est actuellement hébergée dans mon domicile situé à [address].
Vous trouverez ci-joint les copies de nos pièces d'identité respectives ainsi qu'un justificatif de domicile de mon habitation datant de moins de six mois.
Veuillez agréer Madame, Monsieur, l'expression de mes salutations distinguées.