Any company claiming to know anything about Search Engine Optimisation or SEO, should at very least be fulfilling certain functions for you on a monthly basis regarding the marketing of your website. If they are not doing ALL of the following, they are pulling the wool over your eyes, stealing your money, and giving good SEO companies bad names. To get the full impact of SEO, always ensure the following are being attended to:
On-site Optimisation 
The pages on your website tell Google and other search engines what your website is about and what industry you are operating in. It is vital that your SEO company address the on-site elements and make ample use of your keywords in the appropriate places and even stipulate the area you serve to narrow down geographic location. This ensures any person searching for your product/service in your geographic area finds your site. Another SEO rule is that all links on your website should be follow-able by search engines (ie: text, not images). If your SEO guru overlooks this, alarms should start ringing.
Search engine submissions No matter how well you website works or how amazing it looks to the viewer, the bottom line is that if your site is not submitted to the major search engines, it is highly unlikely that you will feature in searches for a long time. Google finds your site using links, and eventually when it follows a link to your site, it will index the pages. To save yourself time, submitting your sitemap to Google, Yahoo and others will speed up the rate at which your pages are added to its search results.
Inbound linking Once your site is adequately SEO’ed, and search engines have decided where your site ranks when compared to other sites competing for the terms, the only way to climb constantly up the rankings in Google is to build high quality, one-way links to your website. This can be time consuming but really is the life-blood to reaching the first page for commonly search phrases. The more reputable the site you link from, the better.
Conversion Optimisation Once your SEO company succeeds in driving high amounts of traffic to your website, the next step is to ensure the visitor is buying something from you or at the very least, that they give you some contact information. Traffic on your site is useless unless you benefit from it, and so is your SEO company if this is the case.
Analytics for measuring SEO success The importance of measuring the success of your SEO efforts is vastly under-rated. The information pertaining to visitor statistics, time on site, pages per visit, which pages get the most attention and why others are getting a high exit rate. All of this is invaluable and can help increase the value gained from SEO. They say knowledge is power, and in SEO terms this is definitely the case. So make sure you evaluate what exactly your SEO company is doing for you, and what value they are actually bringing your business.
Read what others have to say about "fly by night" SEO companies, who promise the world and deliver nothing.
Predatory SEO jerks.