You should’nt just hand over your site to an SEO company and pay them to get you ‘up there in Google’. Best is to get a process in place. Identify the goals (realistic ones depending on your business and market).

Set into motion the parts they will do and how you will contribute to the process. There are areas where the site owner has to start the process and the SEO company will execute elements of it and there are areas where the SEO company has to fully execute all elements. Also SEO has to send recommendations of what site owner can do to further promote the website. You have to ensure these are implemented (once you approve).

Conclusion: Ask them about their SEO process, how they approach a project. See if they ask about your business goals first (which is a must) and your target market in detail (a good sign). Tell them to give a current evaluation of your website and take a second opinion (don’t say an SEO firm did it, say you did it)

Before signing any contract, Check their clientele and ask about their SEO strategies because it may be possible that someone get you top ranking by using black hat SEO for short period

I can suggest one company who has good seo experience and i have practical experience with this company expert i can just share email address for that company.

seomasterinindia@gmail.com contact him i m sure you will get your site on top in major search engine

Try to find a local SEO company, so you can have face-to-face meetings. Ask for references from other clients, and contact them to ask about the companies service. Any serious SEO company will be happy to do this. Beware of companies that guarantee page 1 listings. Ask them to give you a written guideline on exactly how they intend to go about promoting you in the search engines.

Also, have realistic expectations. You need to be continuously building backlinks and promoting your site, and you can wait anywhere from 6 months to a year to see good results in a competitive industry.

The problem you facing is faced by many corporate companies. There are too many people coming in to this industry thinking that it’s some easy way to make money. Reading on-line articles for a month or so many of them think they are proficient in this area. As in any career the experience counts and helps the business in real time.

Coming to your question you could try to below steps and see if you find any luck..
1. Research in different on-line resources like seo consultants directories.
example: http://www.findseo.org or http://www.seoconsultants.com
2. Participate in the forums that are related to you business and see who is offering seo services for your business niche.
3. Once you select few companies call them and ask for a quotation (i assume you have your goals set to achieve from the seo promotion), make sure you to take client references. Call these clients and see if they are happy with the service provider. Visit their website check their website rankings in Top 3 search engines. Remember if they have rankings for moderate to high competition keywords then thats a good sign. If an SEO company ranks their client website for a keyword which is not really used by searchers to find the similar service/product then such ranking is merely waste.
4. Many companies fool their clients by optimising the client website for very low end or less competitive keywords which in the end would never fullfill the business goals in order to make profits. Concentrate on those keywords that get you decent to great traffic but helps your business to get that conversion. Also check the seo company website page rank, alexa rank & their rankings for the relevant and popular/competitive keywords.
5. Last but not least, try to hire a company that is specialised and expertise in providing seo services to your business niche.

hope this helps