Beneath the first post is a great location, not only is it very visible, but I don't find them as intrusive as ads that show in the header, or close to it.
On two of my sites I have a banner to the right of the logo. Not a very big one (~460 pixels wide). It seems to be pretty effective. I get a click every now and then, and impressions from every page view.
I've also tried footer ads, but people actually seen to complain about those more than header ads for some reason.
I would have to say the header is probably the best place just because it gets the best views. I thought about getting a heat map tracking script so I can target the ads to where people view the page the most, but never really got around to it.
Post-article does the best for me in terms of CTR.
Header ads tend to have low CTR as people almost immediately scroll past to read or whatever it is they came to the page to do.
Once they've finished reading the article, placing an ad underneath provides a really nice follow on and prompts more clicks, especially if using Adsense where the ads are often very related to the content.