The most important thing you can do to increase your rankings in the long term is to start developing quality content. You can strategize and optimize forever, but when it comes down to it, search engines love good content (and people do too). If you have something worth linking to, you will attract a lot of natural one-way links, which are the best kind. Also, if you have a lot of textual content on your site there are more keywords for the search engines to find.
A great place to start is to add informative articles to your site that relate to your products or services. Create an article section on your website and commit to writing an article a day or an article a week. Make them relevant to your business niche and helpful to your customers.
Start an online FAQ. Whenever anyone asks a question, add a detailed answer to your website. This will also reduce customer service calls.
Give lots of information about each of your products. This will not only help your search engine rankings, but it will help your business in general, because online consumers love to research their purchases before they buy.
If you have the time and interest, consider starting a blog about your business niche. Blogs can draw repeat traffic, and because most blogging tools utilize RSS feeds, your entries can be syndicated on a lot of other websites very easily. However blogging takes dedication and focus, so think about this idea carefully before jumping in.
Search engines aren't the only reason to build quality content. Having lots of information on your website establishes trust that you know what you are talking about and would thus be a good place to buy from. There is no per-word or per-page costs on a website, so there's no excuse not to add lots of well-written, quality information!