You do not need to place ads on every single page, sometimes most your ad revenue might come from a few high-traffic pages on your site. If that is the case, why not remove the ads on the rest of your pages? Your users will surely love you for it!
Here on Brugbart we normally don't place any content ads, but we also got a few high-traffic pages with thousands visits a month. When we started to place content ads at the bottom of those, our ad revenue more than doubled.
This perhaps just shows that it pays to think about your ad placement. But still, try to avoid placing to many ads in the content. Ads in the content can be quite annoying to your users, so you better limit it to a few carefully selected pages.
Don't just place Ads anywhere, the spots should be chosen together with your web-designer, to make sure that there is a decent contrast between elements on the page - this both lowers the accidental click rate, and makes it easier for users to tell Ads apart from the content.
Ads in the content should be placed with extra care, because you can quickly end up driving visitors away - such ads usually give people a poor impression of your site.