1. You might want to start with split testing:
http://www.optimizely.com
2. If you're running ads, I suggest you start split testing them. Also, re-visit your keyword selection and targeting options.
3. Do you currently have live chat support? If not, get on it. I'd recommend: http://www.livezilla.net OR http://www.livechatinc.com
4. Have you analyzed your site for Trust Signals?
--> http://www.mywot.com (just one example)
--> Are you using EV SSL (green bar)?
--> Do you play any trust / secure logos such as, Verisign Trust Seal or Comodo Trust Seal?
--> Is your company information prominently displayed?
5. Are you using high res, and good quality photos that accurately represent your products?
6. How's your web design look? Are you using the right color scheme for your audience?
7. How's usability?
8. How's your site load speed. <-- HUGE factor when it comes to conversions, just ask Amazon

... they've done extensive research on this.
9. How's your online reputation, do you have reviews? are they good? do they look real / are they real?
10. Is your brand message, brand display, and overall brand image consistent?