Three easy places to start:
a) Simplify the UI - Make it as simple as possible for people to enter their payment details - large fonts, cross-browser tested, minimal pages, optimized for the lowest screen resolution of your average user on up.
b) Reiterate TRUST messaging - testimonials or buying popular symbols (Verign /McAfee / etc) and even dialing in the PLACEMENT of those logos is high-value eg http://www.conversionvoodoo.com/blog/2010/07/proper-placemen...
e) Implement a Cart Abandon strategy - either sending "instant discounts" via email good for 24hrs to complete order, calling dropped carts, or hitting a pop-up of live help or similar is easy money.
Add to this all the usual - proper messaging on buttons, lightning fast response times, mobile optimized version too, etc and you instantly gain a massive advantage over your competitors.