If there's one thing I've learned about upgrading Drupal sites, it's that you're going to have to do the same 50 steps over and over and over again to fix things as they fail. It's awful, tedious work.  Luckily, Drush now has "drush site-upgrade," which makes things considerably easier.  Still it ignores a couple of the really fundamental things like making sure your existing Drupal 6 site is up to date before it starts, and copying the files directory over to your new site.  So I wrote a shell script to do all of this for me.