Improve SEO (Google PR & Alexa Rating)

We are in need of a company to take us from our current Google page rank 4 and Alexa 1,100,000 ranking to PR 5 and under 500,000 in alexa. Our home page has been optomized (title, H1 headings, meta tags, etc) to be SEO friendly, yet we are still not getting the traffic we need. We have about 300-400 current backlinks and our listed in most directories. We are now short on ideas and need a person/company with a proven track record of helping established businesses reach their full potential. We are looking for a long-term provider who can raise and maintain our traffic. However we are open to a 1-time tune up. We could also reach an agreement of a one-time fee and then a monthly fee for services. Please be detailed in your bid as to how you will achieve the ratings we are looking for. Also please include any examples of similar jobs you have done.