Here is a listing of internet filters. Each one works with a different strategy so it might pay to research a few of them or to try a few to see which works best for you.
While there are many to choose from. We would like to draw your attention to http://www.webchaver.org/ and http://www.neto.net.il (this is also known as Internet Rimon) as they both have excellent feedback from users.
For more internet filters: http://www.internet-filters.net/a+.html