If you wait for people to turn around and/or wander away a bit, you will have a better chance of grabbing something undetected. Hide in Shadows (etc) may help with this.
A wizard can use the 'swap places' special move to get an NPC out of a house or shop, and out of the line of sight, then loot the place (wouldn't help for Kistandalius in Ars Magicana).
Any character can use a spider or beetle statuette to chase an NPC away and transform back to loot. Be careful if there are any soldiers or guards around, though.
The easiest method, to the point of making stealing boring, is
to place a container in your inventory. If you open the container in your inventory and then drag and drop items into it, it is not considered stealing. Make sure you are alone when you remove the items from the container, though, because that is stealing.
The lowest weight container in the game is in the thieves' guild in Verdistis; until then a sack of grain weighs 100, and IIRC Lanilor has a 300 weight chest in his house that you can seal before freeing him.
