At one point, Google published a "heat map" on their site showing the best places to put ads. They have since removed it because it made it appear that plastering your site with ads was the way to go. Don't do that, use it to figure out where an effective placement or two for your site would be. Here is a site that still has it and has revisited the question: http://www.vaughns-1-pagers.com/internet/google-heatmap-2.htm
As you can see, the most effective place for an ad is above the fold in the middle of the page right with the content. Ads generally lose effectiveness as you go down the page.
The one exception to this is on article pages. Putting an ad right at the end of the article near the bottom of the page can give the user something to do next after finishing reading.
Unless you have articles and put an ad right after the article, I would recommend putting the ad in a more visible place.