This secret tip should be used very carefully because some affiliate companies aren't too keen on the idea. Be warned, they may want their money back, even though you spent hard earned PPC money.
Just about every time you send a visitor to your affiliate link (example.com/product.html?affcode) they get redirected through your affiliate company to the landing page of the company paying for that visitor. During this unseen transaction a cookie is placed by your affiliate company so they can track your earnings through that customer. Pretty basic.
What happens when a customer comes to your website, sees your great review about widgets from widgets.com but doesn't click your affiliate link? The next day he remembers widgets.com and goes and buys the product. What happens then? You, my friend, get nothing.
However, if you were to preload that landing page in a 1px x 1px square (popup or iFrame) the customer would get the cookie without even clicking your link. If fact, if it worked, you wouldn't even need your spammy looking affiliate code to get credit for the sale!
I would recommend testing this, but in theory it works great and many of the worlds best affiliate marketers know this dirty little secret that you now know!
Try it out and leave some feedback on how it works for you!