At the Vault 21 store on the strip the shopkeeper Sarah has some bubblegum that when picked doesn't leave her inventory. So to get this trophy quickly purchase\find a stealth boy and then wait until she is leaning against the wall. Activate the stealth boy and position yourself by the counter pointing towards her. When it says you are HIDDEN attempt to pick pocket that bubblegum. You will have to back out of the inventory screen each time. Repeat this 2 to 3 times, save game and repeat the pick pocketing and saving until you hit 50.
Might want to make a master save before hand if you want to avoid any karma changes. Works on Ultimate Edition also.