In this work we consider an experimental hardware platform for secret key generation operating in the ISM frequency band, where one of the communication partners has multiple antennas available. For this scenario we derive optimized antenna switching schemes to improve the random characteristics of the channel measurements used as raw data for key generation. We consider different optimization problems to minimize the total correlation of the channel measurements over frequency and demonstrate for real measurement data that with a simplified, low complexity optimization scheme a high or even maximum degree of decorrelation is achievable. The results show that independent antenna switching for each frequency is de facto optimal and that uniform antenna switching probabilities are not optimal. In particular, the results show the effectiveness of the optimal randomized scheme under unfavorable conditions which appear, e.g., in static and line-of-sight communication situations.