Engaging the services of a graphic designer or an agency is as much a financial decision as it is a futuristic one. Consider it as an expense that is undertaken to keep your business in the minds of the people. It should be an ongoing process, especially one that changes according to the demands of the changing needs of the business.
A one-time investment in a designer or even design is not going to give you the staying power in the minds of people. What will make them remember you is how many times you choose to put yourself out there so that our audience remembers you all the time. And you will be surprised at how much people remember good design. If you use a designer to create a website that is functional but also beautiful and then revamp it into something extremely complicated, the chances of your design being trashed are very high.
So when you employ a designer, make sure that they create something that will last long in pubic memory and if you have to retain the same designer make sure that they continue the trend they set!