Luckily, I've yet to have any real problems with thieves. I've had a few cards stolen from time to time back when I was dumb, but I never had a deck or anything stolen. I think the most valuable card I've had stolen was a Delinquent Duo. I also had some guy try to steal my Harpie's Feather Duster and I caught him red handed, it was soooo awesome. Take that you stupid jerk! What now!? What now!? So yeah, I'm very, very careful now, cuz I remember when I collected Pokemon Cards I had a crapload of stuff stolen all at once. My Charizard, and my Venisaur, and more. The guy who stole them also stole my roller blades($$$).
I finally realized why I don't like to trade, because that's when my cards get stolen. Plus, I make some really bad trades. >_<
All thieves should have there hands cut off, just like old times. They can learn to steal with there feet.
I really should go buy a Charizard just to, you know, have it. I guess...