^
Before each of my trips, I bid selected tickets on Yahoo! Auctions with using an intermediate, two weeks (or one ? can't remember) before the concert day...
Yes, I know, it costs A LUNG but airplane ticket too... and going to Japan doesn't happen many times so it must be as secured as it can be.
Well, for me, it's an important part of my budget and planned from the beginning.
Anyway, you can also buy your ticket in shops (the one near Ueno is a good one and there's one in Harajuku too), I remember seeing good seats for the concert I aimed even the day of the performance.