If I can just add a word of caution, you must use an agent that actually has your best interests at heart. Since this in not possible, you must find an agent that is going to get paid the lions share of his profit from marketing the invention for you. That is to say, if the agent requires his entire fee upfront, before he has succesfully marketed your product, you are bound for trouble. The reason is that it's hard to not go home on friday after you have been payed. The agent has already been payed, and therefore couldn't care less if your product sells or not. Now this is not always the case. Some agents will at least attempt to sell your product, however once again, if they have already been payed and they are negotiating licensing agreements that make no difference to them, you do not stand to benefit.

In short, do not choose an "Invention Submission" outfit. They will fill your head full of lies about making money overnight, as soon as you send them the money to prepare a "professional presentation". This hardly amounts to any value at all considering the proper exploitation of a technology, and not nearly what you will end up paying for it. Don't pay big money for patent searches that you can do yourself for free online. Don't believe everthing that a place like this tells you, and be sure to ask them some important questions like, "How many products have you actually marketed succesfully?", "Do you have support from inventors groups, and which ones?", "Are you on the 'good guys' list?".

I am not trying to steer you away from Marketing Agents, simply make you aware of the dangers and ask you to proceed with caution. There are many inventors that have been hurt financially and emotionally from dealing with certain organizations that seem bent on the exploitation of American Ingenuity and that doesn't set to well with me.