Continue Shopping Button

Hi, I am looking to add a "Continue Shopping" button on the product page that will take people back to the "Categories" page, once they have added a product to there cart, I know they can use the back arrow in the browser, but a Continue Shopping would be better, any ideas?
Also what file would the code go in?
Thanks
Rob

I'm not sure whether you have done any template customizations, but usually the categories list will be visible either on the left hand side or as links in header or both.