Simple test case

class MyTest < Watir::TestCase
    def my_test
      goto('http://www.google.com')
      text_field(:name, 'q').set 'ruby'
      button(:name, 'btnG').click
      assert(text.include? 'pickaxe')
    end
  end

Putting it all together

test1.rb:
      goto('http://www.google.com')
      text_field(:name, 'q').set 'ruby'
      button(:name, 'btnG').click
      assert(text.include? 'pickaxe')

test2.rb:
      goto('http://www.google.com')
      text_field(:name, 'q').set 'watir'
      button(:name, 'btnG').click
      assert(text.include? 'Holy Grail')

From the command line:
  > watir -t test*.rb

This would automatically run the tests, creating a testcase wrapper for each test and then executing the combined suite.

Reusing browsers between tests

The "automatic" test suites will reuse a browser between tests, blanking the window, and recreate it if necessary. This is really the best way to use Watir.
* Possibly provide option to close and reopen between tests, but this is hard to make reliable!