I would say it depends on the layout of the site. Stay away from the top of the page as that causes banner blindness, and dont get bent on banner ads.... Try text link ads as well. When placed withiin copy, they can perform much better.
With any form of advertising it is also a good iadea to try and get it to look like its actually part of your website (make it blend) then you look more profesional and people wont think your just trying to advertise to make money off the clicks.
Yes, try to avoid the top of the web site because that will most likely cause banner-blindness. Depending on how your design is made, the placement will vary. Text links are also a very nice way to advertise.