1-)Go to your Social Center Profile
2-)Click "My Home"
3-)Click "My Profile"
4-)Click "Avatar"
5-) Under the "Direct Avatar Editing" clear all parameters under "Avatar Properties except first two of them
6-) Click on "Save Changes" located at the bottom of the screen
7-) Go up of that page to the "Save Current Avatar Outfit"
Give a name like "Blank" or "Ghost" or "Invisible" At the outfit title and click on "Save Outfit"
PS-1: For not loosing your current outfit I advise to do step 7 and 8 at first for your current outfit.
PS-2: You will be invisible but your name will still be shown over your invisible head and you will be also listed at local list and friend list. To hide yourself at those lists you have to hide yourself in-game options.
PS-3: Do all at your own risk.
PS-4: To the other members that read this post: Avatar editing from Social Center Profile is only for VIP members as I know. So if you are basic, you can not reach the mentioned area.