What do you do when you're locked out of your Gmail account for no apparent reason? Google says that since it asks for so little personal information when you sign up, verifying ownership is difficult when they've locked down an account because it may have been compromised. But there's one interesting tidbit they offer for quick account restoration you may not have known:
Always keep the verification number you get when you sign up for Gmail. When you sign up for Gmail, we'll ask you for a secondary email address and then email a verification number to that account. This number is the best way to prove ownership of your account, so be sure to hang on to it.
Most users probably toss that initial verification email, but this seems like a good reason to save it just in case.