Royalty free music sites

Lately I've been looking into selling little bits of music via those royalty free music sites, and I'm just wondering if there are any tutorials or guides about this anywhere. I'm wondering what the best sites are, if there are any tips for being profitable, and other general insider information. I'm particularly interested in what kind of music sells the best, and whether I can release the same track over multiple sites.

Any advice, or info is welcome.