You can start by doing some shopping yourself, and ask yourself what causes you to hit add to cart. :-) Most of us don't even think about it, but if you go into it with this mindset and question every action, some things begin to come clear. Most often it's none of the stuff webmasters put into it.
If I'm already interested in buying, an obvious and prominent "buy" or "add to cart" button. Not just a link with some non-conformist color scheme, not hidden way down below the fold (although, if there is a long description, there SHOULD be another button at the bottom as well as at the top), and not hidden amongst a bunch of animated .gif images trying to grab my attention.
If I'm on the fence, I need some compelling reason why I should buy from you- e.g., great price, free shipping offer, etc. If it's an item I can find many other places, I'd better be able to find out within 1 or 2 clicks how much I'm going to have to pay for shipping.
If I'm not at all interested, pretty much nothing will get me to buy.
Call to action button colours = button.."shiny*"( *vista highlight button style ) "mid blue" #0044CC > on hover #FA8C00 >action
or just "shiny*" #FA8C00 > action
or form button CSS "styled" #FA8C00 > action
It helps if the cursor changes to a hand on hover ( if you are styling form submit buttons using CSS..( without using images for button states ).. you can force "hand" via "cursor: hand".so that the "call to action" gets reinforced..
Why ? ..research it :)..it's not "a mystery" ( what prompts people to buy ) to ad agencies ..nor to psychologists ..nor illusionists /religions etc..just no one who knows tells ( usually )..and sometimes people actually don't like to know ( even when they find out "they will deny it was so" ) what prompts / conditions their actions.
If "it" was known, it would be priceless.
no..but finding out / being told the "why" ..takes a lot of time /money /study ..