People are naturally nervous about buying things online. You're never going to get 100% -- there will always be some people who don't buy from you. But there are things you can do to help make them more comfortable about buying from you, which should increase the percentage of those who complete a purchase.
One of the main things is to increase the credibility of your site. In other words, make it more so that people believe you are a legitimate, law-abiding business.
You could start with the Stamford University Web Credibility Guidelines
. These are ten factors the Web Credibility Project at Stamford found were most influential with web visitors.
You could also read the report put out by Consumer Reports Webwatch
, also on the topic of credibility. You can review an abstract and download a PDF of the full report at that link.
Another thing is to make sure your web usability and conversion marketing are up to par. You'll find a lot more information about those at places such as Marketing Experiments
and in the article archive at GrokDotCom
Then it's primarily a matter of testing. Trial and error. Each time you test, you learn something and you make your site a little bit better.