I found out that there are sites that will do a review of your web page and try and give you a correct estimate of the density of most of the keywords in your webpage which will increase your chances that google places a correct and more relevant add in your page.
I came across this above web page which does exactly that. The limit is 40 k + webpages. Google adsense as a program does not believe in getting blogs to their prorgam as there are high chances that the user will not be consistent with his or her content.
Try and adhere to one or two topics at max. I believe that is one of the reasons that I am getting all public service ads on my webpage since I have been too varied and rainbowish in my choice of topics .