To create gems, install the 'jeweler' and 'gemcutter' gem:

gem install jeweler
gem install gemcutter 

To create a gemspec from scratch:

rake gemspec

To bump the version (a git commit is automatically made):

rake version:bump:minor (or...)
rake version:bump:major

To regenerate the gemspec, push to github and gemcutter, and tag the release:

rake release

To build a local version of the gem only:

rake build

More information at:
http://github.com/technicalpickles/jeweler