Usability tests are a valuable tool - just remember to wear a helmet as they almost always return many head-smacking solutions.
Approach people and have them use your site. Give them a couple common goals, and film them as they go through your site and vocalize their thoughts, surprises and annoyances.
After running a batch of 3-4 tests, you will get a good representation of the snags that your site presents. Many of these snags are a simple fix, and they can yield a huge increase in sales! KaizenSEO will find the test subjects, run the tests, provide video copies, and create a comprehensive report of all the issues and suggested fixes.