Best done at the start of the game when you don't get any notoriety. Just walk past people holding down X and you'll pick their pocket.
You will also temporarily get a carnival mask from Leonardo in memory sequence 9 with will allow you to pickpocket people whilst gaining no notoriety.
If your between or past either these points, you can spend 500f to bribe a herald to lower your notoriety once it is past the 50% mark, then immediately pick his pocket like above. Do this twice and the trophies yours.