Thanks willrax! Well, this was my first project with RubyMotion, so I would have a hard time knowing whether my tips were related to RubyMotion or AVFoundation or Objective C or what.
But one of the things that helped me the most was when I was stuck on something, say on how to set up a AVAssetExportSession, I would search github with that term but filter by ruby files.
Also, because I'm new to Objective C I found working with the nib gem to be much easier than figuring out the OSX/iOS APIs enough to use MotionKit.
And the sublime text OBJC2RubyMotion package was invaluable. It was great to never have to think too much about the conversions, since most of this code came from a variety of Apple's AVFoundation test codes.
All in all, it was frustrating at times but I thought it would take longer than it did to build this app, so I'm pretty happy about it.