I just got my Google+ custom URL today, it's
and I wish I could tell you how I got it, except to say that I have been on Google+ pretty much since it began, I use Google+ every day, and I suppose you could call me a "member in good standing".
I just got a notice that said "claim your custom URL". They picked out the name (which makes sense, it's the name of the account), I agreed to the regular terms that nobody reads, and there you go.
Like all of Google stuff, I would imagine that it's rolling out in stages, so if you qualify (and you should!) just hang on, it will be there soon enough.