Search Engine Optimization:
"SEO" or search engine optimization by a company that "specializes" in it is not something I usually recommend to our clients as simple and low-cost methods such as those mentioned here can be an effective marketing plan.
If you do consider a specialized company for this type of marketing I recommend you get a number of different quotes and exact details and costs of what services will be preformed. I do not recommend anyone re-write your website text for the sole purpose of loading it with key words and phrases. I do recommend you have a professional copywriter evaluate your text so that it is as effective as it possibly can be for your market and that key words and phrases are utilized in areas such as headline, page titles, images tags and your description tag (in your page code).
Google is still the top dog as far as engines go and Yahoo is the top directory site, so a simple focus on keyword campaigns is often much better use of your online advertising budget than paying a SEO company. Google's main qualifiers for ranking include: quality links (links to your site from other quality, well-ranked sites), quality content (it's important that your text is well-written for a human, not a robot), quality page coding (make sure each page has a title tag which can include your key words or phrases, every image has an "alt" tag which can also have descriptive keywords/phrases, and every page has a description tag - again with description words/phrases - your web designer/developer should be able to do this). Here is another good link to read to learn how the "Googlebot" works: http://www.google.com/support/webmasters/
Marketing is Not a One-Time Effort: The last thing to remember is that marketing isn't something you do once and forget about. It requires on-going effort and maintenance. You should be continually evaluating what works and what doesn't while exploring even more new avenues. A few additional web resources you can check out to stay on top of your marketing efforts are:
http://www.jvmediadesign.com/blog/ http://www.wilsonweb.com http://www.marketingpower.com