I get the best results when it:

• is large
• is above the fold
• is very obviously a button
• is to the right of the product image
• explains the action well - Add To Cart and not just something like "add"
• has plenty of room to breath and stands out

When I tried a 3 column layout like Amazon does I didn't have much success, but there's plenty of other things that tie in obviously.

I'm sorry for the lack of numbers but I've been building "ecommerce" sites for 4.5 years. Hopefully it helps and is remotely true!

I get the best results when it:

• is large
• is above the fold
• is very obviously a button
• is to the right of the product image
• explains the action well - Add To Cart and not just something like "add"
• has plenty of room to breathe and stands out

When I tried a 3-column layout like Amazon does, I didn't have much success, but there's plenty of other things that tie in obviously.

I'm sorry for the lack of numbers to back this up, but I've been building "ecommerce" sites for 4.5 years and this is just based on my experience and what I have observed.