Selling ad spaces on a blog/website is the best ways to make money online, when your blog is young and has low traffic you can use some advertising networks, but once your blog has high traffic and good page ranks it is not wise to depend on 3rd party advertising networks to monetize your site. I’ve seen many blogs which have text ads on their side-bar, they sell it at $50 a month, but that blog gets only $25. The above example would have made it clear to you that ad networks eat away almost 50% of your traffic. Click here to read more
Friends say if you don't mind, deal with advertisers directly. Of course, you might also have to sell the ads...