Onlydarksets

Found here.

If you want square pixels, use “-vf scale=-1:-2″, or if you want to scale the height to 480 and have the width automatically selected to get square pixels, use “scale=-2:480″

If you want to add black bars to make the video fit a 4:3 aspect, just use “-vf expand=aspect=4/3″ (then scale to VCD/SVCD/DVD resolution).

If you don’t specifically want either of the above, just use “-lavcopts autoaspect” and it will select the correct aspect ratio for you.