The importance of a power spectral density (PSD) mask restriction is often overlooked when optimizing the spectrum usage for multiuser digital subscriber lines (DSL) systems. However, by developing the optimization strategies based only on the PSD constraints (masks) we can tremendously reduce the computation complexity compared to the methods only based on the total power restriction. In this paper we introduce a mask-based spectrum balancing (MSB) algorithm and demonstrate the near optimum performance of this optimization approach. Furthermore, we show that besides standards compliance, PSD restriction is also needed to ensure the convergence of iterative spectrum balancing methods, which use dual decomposition optimization.