Parenting a player to a rigidbody platform

Hi, I got a player(Character Controller) which should move with a rigidbody platform. When the platform does not have a rigidbody attached, everything works fine when I parent the player to the platform. But with a rigidbody attached to the platform and therefore moving with addrelativeforce the player just slides of the platform, even if he is parented to it. I also tried to attach the rigidbody after the player gets parented(via script) to the platform, but then the player and the platform fall down and I loose control over the player. I also changed the settings of the rigidbody before I attached it to the platform). Does somebody know how to solve this problem?