Ad placement is very important. It is recommended that you have ads above the fold, and in places such as the top left and middle of the page, where more people will see the ads.
What positions work best for you?
At the top of the content on a page is one of the best places to have ads, because its where someones eyes will look first as they reach your web page.
Yeah you can view Google's webmaster blog for better details but they basically say top fold and left sidebar.
It varies from site to site. For a blog, its in the posts (for me anyway), for a proxy its just above the bar where you enter the url and so on
I think placing ads just after a long piece of text works very well, because it makes the reader think the links are part of the content. Just a hint, don't use image ads, they rarely get clicked on
