I'll drop in and give my two cents worth. In my opinion, hiring a SEO company is a worthwhile investment provided you already have a steady flow of business. I personally don't have the time to implement the great suggestions offered by Spinner, which is why I use an SEO firm. They also hire writers that blog on various related subjects, then link back to my site in the credits. For about $1-2k a month, they will get you and keep you in the top 10 search results for the keywords that matter. This often takes 6-12 months however, so its a long term investment. If you're short on capital, why not try Google AdWords instead? You can plunk down $50, choose the keywords your customers use when searching, and watch the visitors roll in. When you pair this with the free Google Analytics package, you'll be able to really track whats happening when visitors come to your website. You can expect to pay anywhere from $.25 - .75 per click with AdWords. If your website is designed properly, you have the product and price the customer wants, then you can expect about 2-4 in every 100 visitors to make a purchase right away. Then you have the folks who do research and come back at a later date to make a purchase, which are harder to track. Google offers free training on their site for AdWords that will help you understand everything about the process in far greater detail than I can provide.
