Methods of reducing a pattern displacement error, contrast loss, best focus shift, and/or tilt of a Bossung curve, of a portion of a design layout used in a patterning process for imaging that portion onto a substrate using a lithographic apparatus. The methods include determining or adjusting one or more characteristics of one or more assist features using one or more rules based on one or more parameters selected from: one or more characteristics of one or more design features in the portion, one or more characteristics of the patterning process, one or more characteristics of the lithographic apparatus, and/or a combination selected from the foregoing.