There seem to be a few different responses from Twitter - some that were successful, some that weren't. Here are a few options to try; in some cases you may need to wait a few weeks for action.
Their automatic reactivation (which used to be at http://twitter.com/account/deleted) no longer functions, unfortunately.
- Email firstname.lastname@example.org from your original account, mentioning your username, and the reason that you want it back now. With the above form disabled, this may no longer work, however.
- Email email@example.com (that's not a substitution for your username ;)) with the username, and a request to start a new account with it - along with the explanation of why you want the username, or why you gave it up.
In either case you may need to wait some time, but it's your best chance with the current methods. If it's a username you didn't own before, Twitter may attempt to contact the original owner about it asking if they want to release it, but it really depends on who receives your email on their team.