There is one law that I have come across in my own study that is the most important law for any business person to know and remember. It is the law of requisite variety. The law states; "the element within a system or the subsystem within a set of systems that has the most flexibility will control the system." Meaning that your business is an "element" or a "subsystem" of your particular business group.

When a business becomes too dependent on outside help to develop ad and marketing campaigns they give up their flexibility, and hence control of the system (competition). 

However your business may be structured, whether you have 5 or 100 employees, do not overlook the creative ability and insight that you are already paying for. Your staff knows your customers better than anyone else. Use that knowledge to your gain. 

I suggest a partnership with an outside ad agency. They have some great ideas and the ability to create many more ad copies and layouts than the average person, they also have a deep pool of ideas. But keep your dependence in check. Bring their ideas and advice into your ad planning, but don't always depend on it. Many times, the persons who already represent a fixed expense on your income statement can come up with great ideas and creative ways to communicate your message in ways that fit with the atmosphere of your community. 

The bottom line: If you want to control your system, stay flexible and give yourself the option of using outside material when you need but retain the option of also using your own creative talent when you can.

This partnership represents a win-win situation for your business. You can take advantage of the experience and expertise of an agency when you want to make that choice, but when you don't want to make that choice you don't have to. By the way, when you do it yourself, you save money too!