When choosing a marketing company it is important to remember that you will need to work with them in order to get the most success out of the marketing they do on your behalf. Marketing companies are experts in the industry but may not be an expert in your industry, so the best marketing is done when companies work together.
Finding the right companies can be tricky as you may be inundated with companies trying to sell you their services. Do not always go with the cheapest option as those that are good at what they do will be happy to justify why they are charging a higher price than others.
Always try and read reviews or use a company that has been recommended to you by others. If you are using a new companies that you do not know much about then try and establish a trial period and do not tie yourself in to any lengthy contracts until you are sure you are satisfied with the work and results they are getting you.