Marketing can be a bit of a mind field to many business owners. You may have a number of companies approach you stating that they can offer you the best service and that you are not taking advantage of marketing opportunities that could increase your business substantially.
With all this information it is very easy to get overwhelmed and end up investing in something that is not going to offer a great return on investment.
When looking for a marketing company to work with, I would always recommend searching locally to start with. The marketing company will most likely have a good knowledge of the area and possibly the competition and will be able to advise you accordingly. Be sure to try and get recommendations or read reviews on a company and ask to see examples of work and results they have gained for other clients. Make sure you understand what they can offer and find out how they track and report progress to you as the client, to allow you to monitor the success rate of any campaigns which they are running on your behalf.