There is a feature with Adsense channels where you can get advertiser's to place their ads directly on a particular ad unit. But your site/blog should be high quality and have high traffic, if you want to be noticed by advertisers. It's called Ad Placement
Dear DP's I did some research on pages for my site. The highest keyword money is for two pages which has content about medication for disease and content on disease itself. The other pages about lifestyle, experience and other things around that disease are getting low paying ads. Secondly I did check in Google account, I can avoid low paying ads but for that I need to know which the sites seerving poor ads are. I can list those websites in ad block. Ads from those sites will be blocked. This seem to be very tedious work to me. Instead of this I went to "search based Google keyword" (http://www.google.com/sktool/#) and searched for keywords which are high paying. I used website name which comes first in ranking for my niche. I think I have got good number of keywords on which I can add new pages for my website. Thanks for your suggestions.
ads robot only show the ads that related with your content tips : keep your post on high payee keywords, make the interest content, post more article