Google rolled out "a new filter that allows you to show your ads only on AdSense sites among the 1000 largest on the web as defined by DoubleClick Ad Planner. This will ensure that your ads reach a large number of users, but only on well-known sites best suited for branding goals," writes Google.
"To activiate, select "non Ad Planner 1000" within "category filters" section of your AdWords account. Keep in mind that not all sites in Ad Planner 1000 are in Google Content Network and that your ads will only show on those sites that're. It's important to note that Ad Planner 1000 doesn't take performance statistics into account, and that by enabling this filter, you may be excluding many sites that're relevant to your advertised products. Additionally, with this or any of our other brand filters enabled, your campaign will run on fewer sites, so you may need to raise your bids if you'd like to maintain your impression levels. We recommend experimenting to determine which feature or combination of features best meets your goals," explains Google.