Just a useful tip i found quite effective:
when you absolutely must put down a bag of anysort whether it be a purse, day pack, or your gear pack, put it down then step you leg(s) into the strap that way if someone tries to quickly snatch it they'll be unsucessful, however if you have the bulk of the bag behind you they could knock you over so be sure to place the bag so the bulk of the bag is infront of you.
Tip for packs
I personally now, put a lock on my backpack, if you are in Australia you might of heard about the Schapelle Corby case, it was about drugs being smuggled in her bag, and she stands innocent that the drugs wasn't her. She has now recieved a 20year jail sentence because of that. It's always better to be safe then sorry.