We study the efficiency of the Atomic Frequency Comb storage protocol. We
show that for a given optical depth, the preparation procedure can be optimize
to significantly improve the retrieval. Our prediction is well supported by the
experimental implementation of the protocol in a \TMYAG crystal. We observe a
net gain in efficiency from 10% to 17% by applying the optimized preparation
procedure. In the perspective of high bandwidth storage, we investigate the
protocol under different magnetic fields. We analyze the effect of the Zeeman
and superhyperfine interaction