What is the best way to export a Motion movie to Youtube?
my 30-min video exports to

This sounds to me that you are not paying any attention to what settings you are using.
What program, what container, what codec, what settings?
On whatever program you are using If possible export to MP4, h.264, AAC.
As you already have rendered the file (assuming it took a long time to render, let's say if it was exported with special effects) another option is to recompress it.
If you are on Windows, you can try VirtualDub2 with the same settings.
You can also use FFmpeg. There are several users that know how to use it well. I'm not one of them :o)
The problem with this second option is that this will be recompressed more times. The initial, this recompression, and probably a third one by YouTube.