Just make sure you're logged into Google and go to https://www.google.com/history. Select "Remove All Web History." This will stop the site from tracking your web history until, and only if, you opt back in.
You should also go to YouTube, which appears to have TWO separate History settings. First, go to http://www.youtube.com/my_history and both clear your history and pause it. Then click on the "Search History" tab from the left column and repeat.
"When a company with as many services as Google is collecting so much information across so many services and combining them into a personal file about you, it naturally raises a lot of questions and concerns," says Ioana Rusu, Policy Counsel for Consumers Union. "Some of the most popular sites on the web belong to Google. If you're online, one way or the other, you're going to run into a Google product. If you don't like the idea of Google being able to collect your activity on all these different sites into a single dossier, there are some steps you can take to minimize the data that Google gathers about you, but the size and scope of this effort are still troubling."