Same here, only 20% of my clicks become a sign up. It just so happens that 20% of my traffic is on firefox 3.0. IE6 and IE7 take up the rest. IE6 and 7 also block 3rd party cookies by default. Maybe this is the reason? What % of your clicks become sign ups? And what % of your traffic is using firefox? (if you know) Also here is a few tips to increase your conversions. People don't want to change websites unless the click EXPECTING to be sent to another website. So if you say: "Buy this product from me by clicking here" and they get sent to a completely different website, they will drop off. If people get sent to that other website expecting to stay on that website, and they have cross to another website, the same thing will happen. So if your strategy is to trick users into thinking that YOU are the merchant (which is what I do) then you need to either mimic your merchant's website, or you have to prepare them ahead of time for the change. Also sometimes your merchant does a really bad job by giving you links that are far away from the offer. You need to take steps to get the traffic closer to the sign-up page. I am having some trouble with this now, as my traffic has to go through 7 different page loads to get there.
Your clicks will convert into sign-up if your visitors have been warmed up to buy the product you're promoting. This is why reviews are very important in affiliate marketing.