Numerical artifacts in the form of spurious oscillations are among the critical issues of Fast Fourier Transfer (FFT) methods for solving multiphase elastic problems such as numerical homogenization, in spite of their computational simplicity and efficiency. In the first part of the present work, it is shown that the irregular discretization of the interface due to the use of a voxel-based discretization is the dominant cause of oscillations. The second part of the present work focuses on numerical artifacts reduction schemes, and in particular special treatments for dealing with the irregular discretization of the interface such as the composite voxel method and neighbor averaging methods. An improved composite voxel method by using the level-set technique is proposed, which alleviates the implementation difficulty of the composite voxel method. This improved method is particularly relevant for non-parametrized interface representations such as those obtained from three-dimensional experimental images.