Joseph certainly knows more than me, but his recommendation is a bit more aggressive than most I've read. For my new Remington 700, I cleaned every round for the first 6, between every 3 rounds for the next 9, then between every 6 for... I haven't gotten that far, but will clean it often for several more boxes.
The gist of it is spot on though - clean A LOT for the first box or so of bullets that go through the gun and then regularly thereafter for several boxes. And use a good bore cleaner. I also recommend a bore snake to make it easy. Also, be sure to initially shoot the same ammo you plan to shoot at critical times. In other words, don't shoot range ammo to break in the barrel, then expect your premium hunting ammo to be as accurate because the barrel is now conditioned to the range ammo.
Update: I just noticed that the question was regarding a pistol barrel. In my opinion, very little barrel break in is required for a pistol. Still clean frequently at first, but I just don't think it's going to make nearly as big a difference as with a high quality rifle barrel.