This year I used a camel back pack (without the bladder) to hold the things I had of value (money, camera, phone etc...) This I brought with me everywhere. Otherwise I locked my bike and put dirty laundry at the top of the bag, tools on the bottom. When in camp I locked my bike and put my panniers in the tent, out of immediate sight under the sleeping bag. If hosteling a combo style lock is handy for storage lockers provided.

This sort of security deters the "crime of opportunity" types but, of course, not the serious thief. i don't think there is much you can do about that other than use situational awareness and avoid high risk areas.