I test my sites on all the major browsers (IE9, Opera11, FireFox5 (or 4?), Safari4) on a windows 7 and xp. And I go from IE7 to IE9 with IEtester.
Then I go to http://browserlab.adobe.com/ and test my sites on all the browsers.
And, off course, I validate my code. And when I use CSS3 or HTML5 I look what browser don’t support something and then I look for a resolve. (with google)