mplayer -vo png movie.avi
mplayer -fs -zoom 1.mov 2.mov
cat movie1.avi movie2.avi | mencoder -noidx -ovc copy -oac copy -o outfile.avi -
This section is not immediately related to mencoder, but some image manipulation can be useful before compression.
The postprocessing tips now lives on the Post Processing page.
Please let me know if you have suggestions or more experiences.