A research plan helps you determine where you're going but it starts with knowing where you've been. If you're doing research yourself, you'll want to create a complete research plan for your problem. If you're hiring a professional genealogist you should start with collecting not only information (names, dates, and places) but the sources you've used. If you don't provide sources along with the information, it is guaranteed your professional will repeat some of the work you've already done. Since you pay for the genealogist's time, you're wasting money.
Even if your research consists only of talking to relatives or looking through keepsakes from the family, you won't get as much value from a professional's time if you don't let them know what information came from these sources. If you can tie specific information to a specific source it's even better but at least indicating what came from talking to family and what you found online is critical. This often answers questions raised by the professional during research.
Recently the Barefoot Genealogist posted a nice quick blog entry about this at the Ancestry.com Blog.
Before you submit your information to a professional you'll need to answer number one above and then repeat steps two and three for each piece of information you have related to what you want to know. The more specific you are, the better research your professional can do. This is true for both what you want to know, what you already know, and how you know it.