Lately I've been noticing a large drop-off rate between the 'add to cart' page and the first step in the checkout process. I'm curious as to how you entice these visitors who have made that initial step to continue through the checkout process. Do you use any kind of incentive or promotion to keep those visitors?
A couple of things that helped me: Make sure delivery fees are shown before they even enter their shipping details. Make sure the checkout button highlights as soon as there's something in the basket so they can't miss it. Make it look like the next steps are easy peasy and secure.
Can you give us the site you are referring to? Generally, you want to make the process as smooth and simple as possible. Things like required registration, high shipping prices, and not showing shipping prices early on are customer killers. Other simple things can help like a progress meter, showing shipping prices in the first step of the cart, very clean form layouts. There are so many factors that go into a visitor making a decision to purchase. If you haven't ever read any of Bryan Eisenburgs articles, I would highly recommend it. I would also recommend both of his books. Bryan's Articles at Clickz - http://clickz.com/experts/author/index.php/19333
Make the layout really clean and get rid of a few external links to stop them distracting the users. The most valuable thing you can ever do when creating or evaluating a site is to stand behind somebody who isn't computer literate and watch how they use and navigate your pages. Do this with 10 people and I guarantee that you will be redesigning your site quite a lot.
I second that. 95% of the time I add something to my cart and then leave the website is because I was unaware of the shipping price early on (and thus the price is no longer worth the item) or because registration is required.