1. Don't carry cash. The merchants all accept your automated ATM card. So stash your cash in the bank. (Downside: bidding on an auction requires cash.)
2. Stash your inventory items inside a container, such as a bag or the pockets of a coat or robe. Stuff can still be stolen from there, but it's harder.
3. Increase your dexterity. The better you move, the poorer a target you make.
4. Avoid obvious crime areas, like the Park NE at night, the Black Plaza at night, etc.
So far, the automated characters who are able to steal from you are all very good at it. (The ones I know of, anyway: Poobah may have others online I haven't encountered yet.) It's not likely you'll notice it happening until you reach higher livels, including higher level See Skill. At this point in the game's growth there's nothing you could do about it anyway. Just resign yourself to occasional losses, and head on back to the money tree for more lucre.