There are two types of commercial ("selling stuff") websites: selling one product, or selling multiple products. You need to use different techniques on each type of site to increase turnover.
Choosing a Product
For multiple-product websites, your main focus should be helping visitors to choose a product from your line of products. Detailed descriptions of products alone aren't good enough. You need to clearly show the differences between products (a "compare products" feature would work fine). Unfortunately, this still isn't good enough. You also need to show the benefits of these differences. For example, showing your visitors the difference between desktop computers; one has 1GB of RAM and another has 2GB of RAM. Unless you tell people what's in it for them to have that extra RAM, people will just buy the cheaper model or, even worse, hunt down a cheaper computer (even it has only 512MB of RAM) somewhere else.
Direct Mail
For websites selling single products, you need to create a direct-mail-letter page. Increasing the turnover rate very much depends on how good the sales copy is. Please take advantage of my archive 
to refresh your sales copy writing skills. Also, make sure there is nothing in the way of visitors reading your sales copy (pop-in windows are a nuisance). The only link leading away from the sales page should be to the payment processing page. (Note: Even so, you still need "about us" and "contact us" pages. Add links to those pages at a corner of the sales page, so the links won't be in the way of visitors reading your sales copy. And preferably those pages open in separate windows.)
Match to Your Site
If the type of site you choose is incompatible with your sales technique, e.g. sales copy in the form of a direct-mail-letter on a multiple-product website, it is very hard to increase turnover. Double-check to see if you are using the right technique.
© February, 2008