Disclosed herein are several methods of reducing one or more pattern displacement errors, contrast loss, best focus shift , tilt of a Bossung curve of a portion of a design layout used in a lithographic process for imaging that portion onto a substrate using a lithographic apparatus. The methods include adjusting an illumination source of the lithographic apparatus, placing assist features onto or adjusting positions and/or shapes existing assist features in the portion. Adjusting the illumination source and/or the assist features may be by an optimization algorithm.