The only requirement is average score... The lower the average, the higher the tier.
Keep in mind, please, that once you are promoted, you can not be "de"moted... So, it is a good idea to record every score and let your natural ability and course knowledge pull you toward the next tier.
People who only post their very best rounds find themselves struggling mightily when they reach the highest levels because they weren't honest with themselves.
Hope this helps you...
Peace.
**EDIT: Check out this thread.