With any ad you run on your site consider the time you put into working on the ad, the space and place it takes up on your site and the results you get from it. Think of the amount of space which shows when your blog first loads on your web browser. That is your ‘real estate’. Are you getting enough back for the space you are giving up. Keep in mind that the space closer to the top of your page is more valuable, just as location, location, location, matters in real estate too. Most ads are not worth the time and space they consume.
Also, if you run a lot of ads you will also lose a lot of readers. Not everyone is eager to hunt and peck for content through a forest of ads. Is your site worth reading or are you just creating a site full of promotion and ad pollution? There is also the problem of having ads which need to be clicked upon. People are becoming so used to ads they no longer see them. That means you won’t be getting clicks.