If you think finding pockets to pick throughout the city when they're relatively sparse in number to begin with is a pain in the rear, You'll like this method I figured out that got me this achievement much faster.
This occurs later in the game so don't push yourself to pick pockets until this if you don't feel it's safe enough at the time. I had gotten through chapter 6 and went to Vittori for his last mission, "Sideshow Attraction". I don't know when exactly you can first access this mission, but you will need to finish his first two before this one is available.
Edit - According to comments below, this mission is available as soon as after completing chapter 5.
When I first started this, I had maybe 40 some total pockets picked. It took me only another four replays to complete this achievement. Add about three more replays if you don't have any at all. That's a little better than an hour depending on how quickly you complete the level.
I noticed almost every guard had multiple pockets to pick in this mission. I ended up with a total of 16 pockets picked and the mission only took a little over 10 mins to complete. I can confirm that replaying missions/jobs, while it does reset the count for that job until completed again, it adds to the overall stat.
The accompanying video shows you exactly how I go about getting through this mission undetected, finding all the loot(only 18), and completing another challenge as well.
You'll see in the video at the end my total stat is well above 100 now thanks to this.
Video will be available shortly. Youtube is processing the final edit.