Regardless, if you use Google products on or after March 1, 2012, you'll give them implicit permission to aggregate your web search history, YouTube viewing habits, and other information Google has gleaned about you into a single mega-dossier. Previously, search histories were siloed and controlled by individual privacy policies, but going forward, all your information will be stirred together into a big data soup.
Although you can't stop Google from tracking you in myriad ways, you can prevent your web search history from being automatically added to your dossier:
1. Log into your Google account: https://accounts.google.com/Login
2. Next, visit the Google Web History page: https://www.google.com/history/
3. If a "No Thanks" button appears, your web history isn't being collected. Otherwise, click the "Remove All History" button, and then click OK on the confirmation prompt.
To see step-by-step pictures, visit the Electronic Frontier Foundation's (EFF) DeepLinks blog post on the topic. A second post documents how to remove your YouTube viewing and search history.
According to the EFF, turning off Google's history features doesn't prevent it from gathering information for internal purposes, but collected data is partially anonymized after eighteen months. Although much has changed in the past six years on the Internet, the EFF article Six Tips to Protect Your Search Privacypublished in 2006 is now more relevant than before.