In P2P file sharing systems, free-riders who use others' resources without sharing their own cause system-wide performance degradation. Existing techniques to counter free-riders are either complex (and thus not widely deployed), or easy to bypass (and therefore not effective). This paper proposes a simple yet highly effective free-rider prevention scheme using (t, n) threshold secret sharing. A peer must upload encrypted file pieces to obtain the subkeys necessary to decrypt a file which has been downloaded, i.e., subkeys are swapped for file pieces. No centralized monitoring or control is required. This scheme is called “treat-before-trick” (TBeT). TBeT penalizes free-riding with increased file completion times (time to download file and necessary subkeys). TBeT counters known free-riding strategies, incentivizes peers to donate more upload bandwidth, and increases the overall system capacity for compliant peers. TBeT has been implemented as an extension to BitTorrent, and results of experimental evaluation are presented.