Look at what you offer and what you're getting traffic on, if the keywords you're found on don't match your product very well, your bound to have a very low conversion ratio.
Hard to help here without a URL or conversion ratio. The last benchmark data I saw for purchase conversion was 4%. How do you compare to that? Without seeing your site I can provide these general comments. 1. Don't try to sell them on the first visit. Offer something for free and require an email address to get it. Then follow up to get the sale. 2. Catch your exit traffic. Give them an offer if they click off your site. They may never come back so that email address is very valuable. 3. Setup split tests on your landing page to see what is and isnt' working. Do a search on Google for "split testing" for vendors that can help. 4. For those that don't purchase after following up via email(several times), try to monetize them by selling them someone else's products(affiliate opportunities) This actually goes for those that purchased as well if you don't have a repeat purchase product/service.