Jan Bath offers up a nice little AutoCAD routine to offset a line — and this one has a twist. Once you pick two lines, this routine will draw the new line midway between the existing two. It's best if the original lines are parallel (or close to it).
Start by loading the LISP routine, then type in the command OL. Choose the line to offset, then pick the second line and the routine will calculate the midpoint. Jan uses this routine to draw a road centerline when the starting objects are both right-of-way lines.