Whoever has had his cell phone stolen knows how frustrating it is to be unable to get his contact list back. To avoid data loss when losing or destroying a mobile device like a PDA or a cell phone, data is usually backed-up to a fixed station. However, in the time between the last backup and the failure, important data can have been produced and then lost. To handle this issue, we propose a transparent collaborative backup system. Indeed, by saving data on other mobile devices between two connections to a global infrastructure, we can resist to such scenarios. In this paper, after a general description of such a system, we present a way to replicate data on mobile devices to attain a prerequired resilience for the backup.