Take note of which NPCs have the steal skill and whether they're directly behind you or not. But the best way is to hammer that button as fast as you can, while you're taking that item.

It works both ways though, I've stolen many items from players and NPCs alike in a similar manner. Time it in such a way that your steal occurs JUST as they touch the box and it's mine!!! Though I must say there have been several occasions where I stole an uber item from a player (Hero's potion/Meteor/Stamina) and it immediately got stolen by another NPC. I'm left as dumbfounded as the person I stole it from.

Steal III has a 90% chance. Yes it still can fail and it's useless when you're in 1st place. I think the way the NPCs use it is fine.