Raise your Pickpocket level, or choose lower character level targets (IIRC, Tingalf in Ars Magicana is easy to pickpocket). I think standing behind someone or sneaking helps your chances, as well.
If you want an exploit that will make stealing very easy;
<span class='standouttext'>Spoiler : </span><span class='spoiler'>Place a container in your inventory and open it, then drag and drop loot into the container (I believe this works with Pickpocket). This is not considered stealing, so you can simply walk away. It is considered stealing when you remove the items from the container, though, so make sure you are alone when you do so.</span> (highlight the text to read)