here is my way of encoding to amv:
install gordian knot (go to http://www.doom9.org)
install vfapi (you can also get it at doom9)
load an avi file into gordian knot, set the desired resolution, the cropping and filter settings (maybe deinterlace, maybe noise reduction if neccesarry), then save the preview window as a .avs file.
open the .avs with notepad and alter the settings like you wish (i added msharpen.dll for sharpening the video (increases the look of the video al lot! and replaced the lanczos resize filter with the lanzcos3 filter (very optional step though, not much of a difference)
of course you have to get the desired avisynth plugins/dlls at avisynth.org and put them all in the .avs file.
now: open up vfapi and load the altered .avs file. it will save a .avi which is a reference to the .avs file with an added avi header.
advantage : you can have all the resize, cropping, sharpening procedures but don't need to re-encode your source video for that and then have it again re-encoded to amv by the amv converter. you save one encoding step, and also the quality of the resulting video will increase because you don't have to encode your video once before handling it to the amv converter.
this procedure is called "frameserving" of a video file. you could also follow the virtualdub method of adding filters to the source video posted in the board and then have virtualdub frameserve the video to the amv encoder, but the frameserving of virtualdub is somewhat strange and doesn't work for me.
the resulting video file will significantly increase in quality with the pre-enchancements and with this method you can frameserve any video filetype to the amv converter as long as your media player can play it back.
this was a quick & dirty tutorial, maybe i will describe it better when there is interest.