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You have to have very high blog traffic to be considered by advertising networks (at least a few thousand visitors per day) but if you qualify, you can depend on a more steady income coming through well-established advertising corporations that usually pay handsomely to place ads on your blog. You can also provide information directly about your blog and select what type of advertisements you want shown. Here are some advertising networks that you can look into for adding advertisements on your blog.  Blogads BuzzMachine Federated Media TechCrunch
Pick specifically which ads you want to display on your blog through advertising networks.