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In this paper, we study the performance of various amplify-and-forward (AF) relaying strategies in terms of ergodic capacity and bit-error-rate (BER) for single-user MIMO channels with single and multiple relays. We first consider some hybrid methods for the single-relay channel and compare their performance with previously proposed methods under various signal-to-noise ratio (SNR) regimes. We then extend these hybrid methods to multiple-relay channels and unveil that they can exceed SVD-based optimum methods in this case. Finally, we propose new multiple-relay strategies based on a cooperative minimum mean square error (CMMSE) criterion. Through simulations, we show that the new hybrid methods can outperform existing ones in the multiple-relay channel, at the price of a slight increase in implementation complexity.