When it comes to choosing an SEO company, there are several factors to consider. There are many different types of SEO companies offering a range of services, some of which are more effective than others in actually increasing search rankings.
One thing to avoid when choosing an SEO firm to provide optimization services is any company that simply wants to commit you to a monthly plan of "directory submission". While directory submission is part of an SEO process and can be very helpful, typically the companies that focus exclusively on this kind of inbound link building are going to be very cost effective, but almost entirely ineffective at sucessfully optimizing a website.
Your SEO company should be familiar with web development, and should give specific and detailed direction with regard to changes that need to be made to your website structure, and content that needs to be created or updated. Having a site that is easily indexed by engines such as Google, MSN, an Yahoo is key to any good SEO plan.
Finally, the SEO company you choose should have a working knowledge of Internet marketing techniques, and should be willing to work with your marketing department or outside firm (if you have one) in order to ensure that brand consistency is maintained across any social networking, press releases, or aggregate news sites that are being used to help spread the word about your company