53415 SEO plan for wholesale store

Our website is 7 years old, OSCommerce based and I have taken care of the SEO from the very beginning. We just dropped from PR6 to PR5 so I am started working on links. We are getting around 1000 - 1200 unique visitors per day and it seems like we can't break through that amount. My goal is to double our visitors over the next 12 month so I need a very experienced SEO to carefully go through our site and make a plan for it to happen. Please note that I do consider the site as optimized and we do have many top listings in Google, Yahoo and MSN. I am sure however, that something can be done to take it further. The plan should contain all steps in details including the specific reason for each step. The whole plan should realistically make our site reach the goal. I am not interested in any PPC suggestions and as I mentioned we are already working hard on linking. No black hat (spammy) plans please. I am only interested in the plan. I will make the actual changes on the website myself.