This is a must read before finalizing your Search Engine Optimization consultant - http://www.google.com/webmasters/seo.html

There are ways to get fast track ranks in search engines, by exploiting the loopholes in their algorithm. Few amateur SEO service providers try to deceive (spam) the search engine spider by means of hidden text, deceptive cloaking or doorway pages which compromises the quality of the results and bring better results for a short run. But they carry the risk of being noticed by search engines. If a website is detected for spam, search engines will penalize by removing spammers from their index immediately, so they do not show up in search results at all. We do not want to discuss these spamming techniques here, since we strongly resist using any such methods.

You should be careful enough to consider the following guidelines to find the credibility and authority before hiring.

Check the physical existence and reliability of the firm   Check does the firm have a physical address, landline telephone, and their experience must be considered     Check the ranking of existing clients   Check the URL of existing clients, check the standard of ethical SEO practices and ranking achievements.     Be aware of promises   "We submit your site to hundreds/thousands/lakhs of search engine and Continuous search engine resubmission on monthly basis". This is totally a junk statement. No fair SEO optimizer would seek to sell their service by promising false proclamation. As said, there are no thousands of search engines, there are only few major search engine and directories which should be submitted manually. Once your website is listed with the major search engines there is no need to resubmit.     Terms and conditions of payments and services   Be cautious of vague contracts in services and payments, don't fall prey to vague and simple search-engine-marketing unethical practices. Obtain a methodical contract from an SEO firm, one that charts services, specifies work ownership, provides termination provisions, non-complete clauses, non-disclosures, payment terms, legal terms, etc.