Aside from making sure that you let your hair grow for a while to make it easier for the hairstylist, the best thing is probably to get as many different shots (preferably HQ) from many different angles as possible. From the front, back, left, right, from above looking down, etc. The more varied the pics, the stylist will have a better idea of how the overall look is supposed to be.
Also, obviously, make sure that you go to a stylist that you trust has the skills to pull it off.