require 'lib/some_rake_file' # or require 'lib/some_rake_file.rake'

=> rake aborted!
no such file to load — /home/your_project/lib/some_rake_file

Rake assumes the ‘require’d files end in .rb, so it won’t find your .rake files. You need to import rake files:

import 'lib/some_rake_file'

This is fine for individual files, but I wanted to include all files that end in .rake in my tasks dir:

Dir.glob('tasks/*.rake').each { |r| import r }

…and there you go. While dead simple, this exemplifies an important distinction between require and import that I found to be poorly documented. Keep your rake tasks organized and remember that Rake isn’t just for rails apps!