Byron

MRH's very own Byron Henderson http://layoutvision.com/

Top of the line when it comes to track design and an over the top all around good guy.  You'll want to read his vast amount of articles to help you better through the process like http://layoutvision.com/id51.htmlor these http://layoutvision.com/id9.html.

Steve

Contest

You could take the route that Rob Teed did, and hold a contest.  He put up $100 for the best design which fit his space (which I won ).  Like you, he was not comfortable with his ability to develop his own track plan and was willing to pay someone to do it for him.  He seems happy with the result.

Whether you come up with your own design, or hire someone to do it for you, you'll need some basic information.  First, of course, is the exact size of the available space and desired scale.  Next are your list of things which you must have and things which you'd like to have - your Givens & Druthers.  How many people will be operating there at one time?  What sort of operation do you want (switching, main line running, trains in scenery, etc.)?  Do you want to be able to switch cars in a yard?  Service locomotives?  What era and locale?  There is a lot to think about, obviously, but doing your homework before hand will greatly increase your chances of having a layout that you'll be happy with in the end.

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I second Byron.  He's worth the expense.

LayoutVision

I also recommend Byron. I didn't have him design my track plan but I did employ his services for review of the plan once I was finished. His observations, suggestions, and comments were invaluable. They kept me from making mistakes I would have regretted later on. Well worth the expenditure.