If I buy a banner ad frrom a very high traffic site, should I make it a NO FOLLOW ad? I read this somewhere. If so -- how can I make it No follow?
No you don't want to make it nofollow or you won't get any seo benefits, except maybe from Yahoo. A while back Google told webmasters to make any sold links nofollow. This was because Google didn't want people's PR to benefit from paid links. Google just didn't have a good enough way of finding all paid links themselves, so they resorted to asking webmasters to do the work for them.
you generally don't have any say, it's up to the advertiser but if you did have an option, which I don't think you will, you would not want the nofollow attribute added
Google wants you to follow it, but they can't expect everyone who has ever bought a link to suddenly switch their links to nofollow. That is ridiculous. If they were to enforce this 'rule' more, all they could do is penalize the site selling the link. They could not punish the site that bought the link.
In most cases you will not have a say in the matter when it comes to posting the banner on an advertisers site. Some advertisers will work with you on what you want the alt text to be as well as making it no follow. I would not worry about it, banner advertising has been around for years and google does not see it as a backlink anyhow. They only see a backlink from a text link, not a banner image.