Don't stay logged in! Search engines want to know who is using them. For that specific reason they have developed lots of different programs which require a login, and provide them for free. This then means that if you search, all searches are allocated personally to you.
To prevent this don't use the same web browser to search if you are logged into their services. Good practice is, for example, to have two web browsers, ie Firefox and Internet Explorer, Firefox being used freely for searches and not logged into any sites, and Internet Explorer being connected with your IDs to the services you specifically need.
All searches you make on the internet are tracked for marketing, or other purposes, if they can make money. Whilst most of our online searches will not pose any problems at all, be aware that searching for medical or legal information may inadvertently be added to your online profile, especially if you are logged in and they can personally identify you.
Another option is to use a search engine which does not disclose your IP address, such as StartPage.
Plugins like Ghostery for Firefox also prevent tracking of your activity.
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