If you can remember your email that you used for that account you could go back into your old emails and locate an email sent by Second Life. You can find your name in old abuse report emails, welcome to second life emails, offline IM emails, etc.
You could also try going down the alphabet in your head and coming up with letters that could be familiar. Or you could try thinking of what you were thinking when you were going to create your account. For instance, when I was going to create my alt's name, I thought Olivia, then I thought Oma, Oliver, Olga, then finally I thought of Olive because it was the name of the little girl in Little Miss Sunshine.