If you are entrusting your marketing to a third party, you will need a way of checking if they are doing a good job and if the money you are investing is being spent well.
Most good marketing companies will give you this information without having to ask for it. So for example they may produce a monthly report which shows the traffic to your site and the position of some of your top keywords on the search engines such as Google or Yahoo. This will allow you to track progress and see areas which are improving and areas which are falling behind. Even though they are doing your marketing for you, you still need to have an input in to it and be sure that they are focusing on things you want them to. I
If for example, you have a new product due to be launched then you need to let them know about this with plenty of notice so they can consider how they are going to tackle it. If you are only willing to invest a small amount of money in to your marketing then you cannot expect amazingly quick results. The more you invest the more you will get out, if done in the correct way.