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The new FTC rules concerning "word of mouth" endorsements do not expressly state how you should disclose the connections.
See a summary here: http://ftc.gov/opa/2009/10/endortest.shtm
Basically, the rules state that if you are giving a testimonial or endorsing a product or site, you need to tell your readers about any payment or relationship you receive for recommending them, as long as the connection is something unexpected.
For advertisements, you don't have to do anything. If you are linking to a product, you don't have to do anything. But if you give a testimonial about how great the product is, and you are being paid to give that testimonial, you need to disclose that. But there is no certain verbiage required. I think anything that explains that the testimonial is not total impartial, and that you are receiving something in exchange would work.
I would think that would at least protect you from getting fined. Like I said, it does not exactly state what needs to be done, just that you have to "disclose" it. I would think any warning would work. The issue is that "affiliate link" does not really have any meaning in and of itself. So you might put and asterisk * by it, and then at the bottom of the page a more full explanation. This way, there is no disruption for the reader, and you can explain the relationship clearly.