Username Availability: A Game of Luck or Strategy?

I have been in search of a good username that I would be happy with keeping without changing it a month or two down the line. I have noticed that many people I've associated myself with in the past have had incredible luck finding display names; not one, two, or sometimes even three but up to a hand-full of names. I'm wondering if there is a certain strategy to looking for available display names or is it purely luck? I've got my eye on a few that no longer register on the hi-scores or by adding them to your friends list but still they're unavailable for take.