...and continuously, and consistent way, let the computer do it for you.
gem install ZenTest
gem install autotest-rails
gem install redgreen
If you're on MAC:
gem install autotest-fsevent
echo "require 'autotest/fsevent'\nrequire 'redgreen'" >> ~/.autotest
If you're on GNU/Linux
gem install autotest-inotify
echo "require 'autotest/inotiny'\nrequire 'redgreen'" >> ~/.autotest
just run 'autotest' from your project root directory, and it will run the relevant tests on your modified files.