Have a name for yourself that you think is really cool? Want all of your friends to call you by it? Follow these steps, an excursion into reverse psychology, and see how it works for you. I myself did this unintentionally, that's how I know.

[edit] Steps

Choose your nickname.
Put it out into the public domain; if you have messenger, myspace, etc., you can replace your normal name with it.
Here is the key: When people ask what it is, get all defensive about it, remove it as your username, and tell them it was pretty bad in retrospect. Get angry at them whenever they use it. You really need to hate the name, and curse yourself for coming up with it. This is possibly the only way to get people using a nickname quickly and easily. Unless your friends are so nice that they'll actually listen to you, it should work.
Gradually come to accept the name, and allow it to be spread by the original users of the name.

[edit] Tips

If the name references something obscure and funny to the people to whom you give the name, they will use it more, especially if you say that the particular reference was unintentional and undesirable. The key is to let them think they have a surefire way of annoying you.
Choose your target carefully. A person who is more likely to annoy you is more likely to fall for the aforementioned trick.

[edit] Warnings

Search your new nickname on Wikipedia and Google before using it, and see what else is using it. It might be a porn site or some crazy cult. On the other hand, these may be desirable, as aforementioned in Tip #1.