Most of the marketing people I see on this list post things that are either completely obvious or highly questionable. Some keep suggesting they are teaching "secret" information and valuable tips. For example, I don`t know how much anybody needs to be told "make sure the message on your website it clear" or "try to be better than your competition."
What i would look for is results. Nicole is right, you could be working with a top notch Marketing firm who has some great clients and have some great case studies but it may not be the firm for your companies needs. Start on a small contract see how it goes result wise and then make a decision if they are right for you.
When I first started my business, I was able to utilize a colleague who had her own marketing boutique firm and boy did it pay off! She created press releases and and also due to her contacts, my product was even featured in major magazines and local newspaper.
The cost was quite affordable as she alloted me a few hours per months. If you would like her contact information, please contact me as she`s worked with all sorts of industries and has a proven track record.
To piggyback with nhgnikole`s comment,...it depends on what you are wanting a marketing company to do?
Maybe you are looking for help in positioning your company?
Are you wanting help in branding a product?
The best channel strategy? And how you will avoid conflict.
Determining your target martket?
A pricing strategy? Do you sell on lowest cost or features and benefits?
Or a promotional strategy which includes; personal selling, advertising, tradeshows, sales promotions, press releases, etc.
Or a company that can help you to develop and execute a complete Marketing Plan that contains all of the above.