So here we'll focus on ad placement. Below is a diagram or heatmap put together by Google. The darker areas are the 'hotter' areas, the places where ads get more clicks. As you can see they are the left sidebar - this is often where people look straight away for a navigation bar. Just below the header is also quite popular. The left of the main content and above main content get the most heat. You'll find, although it differs on certain webpages because of layout, styling etc, that peoples eyes will follow roughly the same route around the page. And as shown on the diagram the left of the page and by the primary content are the best places for ads. Finally the area beneath the main content is great advertising real estate. This is because visitors read the main stuff then are looking for somewhere to click. An ad unit fills this space perfectly.
As well as ad placement, ad styling is important as well. To go back to the previous analogy a poster that grabs attention yet pleasing to the eye is the most read. Now this is something that is unique to each site. The ad placement is fairly uniform for most sites but this isn't. You've got to fit them in with your site's colour scheme or make them loud and brash to catch peoples eye. You would think loud bright ads would get the most clicks but in fact it is actually those which blend in with the colour scheme the best which have the highest CTR.
So remember all this when you're placing ads and you should see an increase in your overall CTR and therefore more of the moolah should come your way.