Tips, tricks, and troubleshooting

The video looks okay, but how can I give it that last little push?

     It's never a good idea to offer one-size-fits-all solutions, and I hate to suggest there's only one answer to this, but I find GrainFactory3, by Didée (the TempGaussMC guy, go figure; I've lost track of all the great scripts he's produced), and AddGrainC, on which it depends (download, discussion), really help sell the upscale. A little noise in the right places works wonders, and GrainFactory, even at its default settings, makes everything seem sharper. It's psychological, to be sure, but it's exceptionally effective.

Youtube doesn't like Lagarith files, what else can I use?

     There's always XviD, which is reasonably efficient, but don't dismiss x264vfw before giving it a shot. I know no one likes using compression with B frames inside of AVI containers, but I've had excellent results feeding h.264 in AVIs to Youtube. If you use this codec, be sure to check the configuration to see that "VirtualDub Hack" is enabled.
     For any codec, however, I have another recommendation that should help alleviate the stuttering so common with HD on Youtube. Set your keyframe interval ("Max GOP size", whatever it's called) to the largest factor of your framerate that's 30 or below. That is to say, use these values:
     Honestly, although I know what a Group of Pictures is, and possess a layman's understanding of interframe compression, I can't say with certainty why this helps Youtube produce smoother encodes. All I can tell is what I gain from empirical results of my own tests, and this seems to help.

Error message: "AVISource: couldn't locate a decompressor for fourcc dvsd"

     This error is an old standard, and is the result of not having a system-wide DV codec installed. Most NLE packages, large and small, have internal decoders for the DV25 format, and can play the footage without issue. AviSynth, however, uses the Video for Windows architecture, and in order for it to decode DV, or anything else for that matter, a VfW compatible codec must be present on the system.
     For DV, a very common recommendation is Cedocida, by Andreas Dittrich. I'm a user myself, and I'm quite happy with the results of both decoding and encoding. Donald Graft has compiled a Windows version, which can be found by heading to http://www.neuron2.net/ and checking the "Hosted" section. At the bottom you'll be able to download version 0.2.2, which is the most recent.
     Extract the files to a convenient location, right click the .inf, and choose Install. That's it! Just don't forget that when using Cedocida to compress widescreen DV, you must go to the codec configuration dialog and set "Aspect Ratio" to 16:9.

Green, rainbow-blocked, or otherwise glitchy frames in VirtualDub

     Frankly, my education in the field of video compression is insufficient to properly identify the cause of this one, but one SimpleSlug user has reported to me that setting VirtualDub to Video->Full processing mode and turning on "Preserve empty frames" solves this. I can't say why that works, but if you find yourself facing the problem, I'd suggest trying this. If it doesn't work, try disabling multi-threading (delete, or comment out, the SetMTmode line in your script). Things will run much more slowly, but you've got to do what you've got to do, and turning off MT is a good general purpose problem solver.