Physical layer secret key generation exploits the inherent randomness of wireless channels. However, for wireless channels with long coherence time, key generation rate can be extremely low due to lack of randomness in channels. In this work, a novel key generation scheme is proposed for a general wireless network where channels are static and each transmitter is equipped with two antennas. It integrates opportunistic beamforming and frequency diversity to achieve fast secret key generation. In this scheme, channel fluctuations are first induced by controlling amplitude and phase of each symbol in the training sequence on each antenna. Thus, key generation rate is significantly increased. However, the induced channel fluctuations lead to correlation between the legitimate channel and the eavesdropping channel, which compromises key secrecy. To this end, frequency diversity is then exploited to ensure that key secrecy grows with the key size. The secret key generation scheme is investigated in both narrowband and wideband systems, and its performance is evaluated through both theoretical analysis and simulations. Performance results have validated randomness and secrecy of secret keys and also illustrate that the proposed scheme can generate secret keys at a rate of 2Kb/s for narrowband systems and 20Kb/s for wideband systems.