Guard-behat will run behat tests every time you save a file. Launch guard and start working. If you do TDD (Test Driven Development) you will get a very short feedback loop.
guard-behat has not been pushed on RubyGems yet. If you want to try it, clone the repository, build the gem and install it.
git clone git://github.com/EricHogue/guard-behat.git
cd guard-behat
gem build guard-behat.gemspec
gem install guard-behat-0.0.1.gem
Please read Guard documentation.
You can get a default Guard file by running $ guard init guard-behat