Be aware that most so called Search Engine Optimsation companies are nothing of the sort.
Most SEO companies know the basics, which are listed on this page. Once the above methods are exhausted, most have no idea how to make further progress apart from with their invoicing techniques.
There are four main things to look out for.
"results or you only pay half our fee"
If a SEO company says "results or you only pay half our fee" then what they are really doing is guaranteeing that they get half the fee for sure (in advance). If you are unhappy then they already have half of the fee and you have no room for complaint other than not to pay for the second half.
Guarantees that you will be on the first page on at least three search engines. If anyone says this, ask them which three search engines. If the list does not consist of 3 from the following: Google, Yahoo, AOL, MSN, Overture or Alltheweb, you are wasting your time and money listening to any further talk about Guarantees. Some of the sharper operators have three 'search engines' which they have made themselves and of course on the day they check, you will absolutely be on page 1 of all three.
The second thing to watch out for with guarantees is what keywords/phrases they are making the guarantee for, If you are a pension company called "Adrian Parkers Pensions (Scotland)" and they promised you will be on page 1 with the keyphrase "Adrian Parkers Pensions (Scotland)", then you will find that once you submit your own site to Google and your site has been scanned, you will always be number one with that unique phrase. In this example, you should state that the keyword/phrase to use is "pension" or "pensions Scotland". Now that is a much harder proposition to make work.
The SEO company not explaining in plain English what they are doing with and for your site.
If an SEO expert starts to talk about link popularity and special algorithms only they are aware of and starts to talk about normalised vector database techniques etc., ask them to explain it in English. If they can't, it probably means they have read a document somewhere and have not understood it. You should not trust a company that is suggesting they can make it easy to get your site up in the search engine rankings if they can't make it easy for you to understand how.
If an SEO company starts to come up with statistics about search engines and usage of search engines, be sure to ask for the source of the statistics. We have had so called experts call us and offer their services and as the conversation progressed, the statistics chaged every time we disputed their numbers, and no sources were forthcoming. I.e. the numbers were made up to suit their world view.