Choose SEO Company

Check the PR of their own site. If they can't optimize their site well enough to get a good PR (over 4-5), they are not worth hiring.

Ask them what keywords their site ranks for. Similarly to the page rank factor, if they don't rank well for the keywords of their choice, they are hardly as professional as they are pretending to be.

Do they use automated submissions? If they do, stay away from them. Automated submissions can get you banned from search engines.

Do they use any black hat SEO tricks? You need to know in advance what black hat SEO is in order to judge them, so getting familiar with the most important black hat SEO tricks is worth before you go and start cross-examining them.

Where do they collect backlinks from? Backlinks are very, very important for SEO success but if they come from link farms and other similar sites, this can cause a lot of trouble. So, make sure the SEO firm collects links from reputable sites only.

Get some personal impressions, if possible. Gut instinct and impressions from meetings are also a way to judge a company, though sometimes it is not difficult to get mislead, so use this approach with caution.

High price does not guarantee high quality. If you are eager to pay more, this does not mean that you will get more. Just because a firm costs more DOES NOT make them better SEO's. There are many reasons for high prices and high quality is only one of them. For instance, the company might work inefficiently and this is the reason for their ridiculously high costs, not the quality of their work.

Cheap is more expensive. This is also true. If you think you can pay peanuts for a professional SEO campaign, then you need to think again. Professional SEO companies offer realistic prices.

Use tricky questions. Using tricky questions is a double-edged sword, especially if you are not an expert. But there are several easy questions that can help you.
For instance, you might ask them how many search engines they will automatically submit your site to. If they are scammers, they will try to impress you with big numbers. But in this case, the best answer would be "no automatic submissions".
Another tricky question is to ask them if they will place in you top 10 for some competitive keywords of your choice. The trap here is that it is them, not you, who chooses the words that are best for your site. It is not that probable that they will choose exactly the same words as you suggest, so if they tell you that you just give them the words and they push you to the top, tell them “Goodbye”.

Do they offer subscription services? SEO is a constant process and if you want to rank well and keep on like that, efforts are necessary all the time. Because of this, it is better to select a company that includes post-optimization maintenance, than get a company that pushes your site to the top and then leaves you in the wild on your own.

We tried to mention some of the most important issues in selecting a SEO company. Of course, there are many other factors to consider and each case is different, so give it some thought, before you sign the contract for hiring a SEO company.