Do it once in your text.

And then I would do it either on the side of the text, with the text wrapped around, or below your content.

I would really split test all of them, but I would start with one of the above.

I place an affiliate link at the top, middle and bottom of each product review. I've found that text links have much higher CTR's than image links. I don't even waste my time with buttons anymore.

The secret is to have good reviews that keep the visitor engaged, and then hit them with a strong CALL TO ACTION in your text link.

For example,

Click here to read full customer reviews at Amazon.

My site isn't a review site though. Currently on each post, I place 1 link at the top, 1 at the middle, and 1 on the bottom; 4 other little image + link ones are on the right sidebar.

Read through your copy and see when you are "ready to buy." You could put the "buy now "image right there. But, put links in the beginning, middle, and end (wherever you "buy now" image is not). So, you'll have a total of 3 click out opportunities. Those links can be text links so it doesn't look spammy.

Of course, there is no set rule as to where the best place is. Only testing will reveal that.

Hope that helps.

Charlie

I usually include a buy button beneath the product image along with a call to action link after the intro and another call to action at the end of the review. I also link the image as well.