This used to be much more complicated, but nothing felt as solid as this core setup.

Gemfile

...

group :development, :test do
  gem 'rspec-rails'
  gem 'capybara'
  gem 'spork', '~> 0.9.0.rc'
  ...
  gem 'rb-fsevent' # recommended for non-polling filesystem event notifications
  gem 'growl'
  gem 'guard'
  gem 'guard-bundler'
  gem 'guard-spork'
  gem 'guard-rspec'
end

Guardfile

guard 'bundler' do
  watch 'Gemfile'
end

guard 'spork', :cucumber => false do
  watch('config/application.rb')
  watch('config/environment.rb')
  watch(%r{^config/environments/.*\.rb$})
  watch(%r{^config/initializers/.*\.rb$})
  watch('spec/spec_helper.rb')
end

guard 'rspec', :cli => '--drb --color --format documentation' do
  watch('spec/spec_helper.rb')                       { 'spec' }
  watch('app/controllers/application_controller.rb') { 'spec/controllers' }
  watch(%r{^spec/.+_spec\.rb})
  watch(%r{^app/(.+)\.rb})                           { |m| "spec/#{m[1]}_spec.rb" }
  watch(%r{^lib/(.+)\.rb})                           { |m| "spec/lib/#{m[1]}_spec.rb" }
  watch(%r{^app/controllers/(.+)_(controller)\.rb})  { |m| ["spec/routing/#{m[1]}_routing_spec.rb", "spec/#{m[2]}s/#{m[1]}_#{m[2]}_spec.rb", "spec/acceptance/#{m[1]}_spec.rb"] }
end