You can inspect the person's level before attempting to pickpocket them. Then, you need to follow them and continue stealing from them. The closer you are to the minimum level for that person, the harder it is to pickpocket. Then, it gets exponentially easier. For example, if you are level 1 and the person is lvl 1, then you chance of succeeding may be 5%. If you are level 2 and they are 1, it may be 15%, if you are level 5, maybe 50%. It gets easier and easier. So if you turn level 10, you may not want to rush to a level 10 person just yet. Level up a bit first. I don't call it grindy for nothing! It's a grind, especially at the early levels. Push through!