In this paper, we propose some preprocessing techniques that improve the performance of the LSF based method for DOA estimation in the presence of rank deficiency in time correlation matrix frequently occurred in practical application. In these methods, the degrees of freedom are reduced so as to fulfill the ill-conditioned optimization problem with several schemes such as temporal smoothing, subspace constraint, DFT beamspace transformation and diagonal loading being taken. Simulation results verify the efficacy of the proposed modifications.