In this paper we develop a power allocation scheme for a single-relay system applying Decode-Forward (DF) based on the resulting bit error rate (BER) at the destination. First, an analytical expression for the BER for M-QAM modulation considering estimation errors at the relay is derived. Based on this expression, the total transmit power is optimally assigned to the source and the relay in order to minimize the probability of errors at the destination. The preciseness of the derived closed form expression as well as the superior performance of the proposed DF-based relaying system are demonstrated by simulation results.