Instructions
Combine your name with an interest or hobby. Instead of just "Dave" you could be "HockeyFanDave" or "SkateboardDave."
Use non-letter characters in your gamertag. The name check takes numbers and symbols into account, so you could use "x_X_x-Dave-x-X-x." It's best not to get too elaborate, since too many characters makes your gamertag hard to remember and difficult to type.
Take the first two or three characters of your first and last name. If you don't like the results, try different combinations, such as the first two digits of your first name and last three of your last name.