Get luggage locks!
The last time I did a world tour in 2011, I was hauling 60 kg in four bags. Two of the bags were checked whenever I flew, and all four items were occasionally in the care of a hostel front desk or a hotel valet. I remember I noticed a few things missing, although it is possible that I simply misplaced them. The total value of the lost items was bearable, but the wonder if my possessions had been violated was unsettling.
Earlier this year, I decided to mitigate any stress over theft by purchasing these luggage locks. I feel much better about leaving my bags unattended in a room. Most theft are crimes of easy opportunity so seeing a tough lock on a bag will immediately suppress all but the most hardened criminal instinct. These locks are airport-approved, although luggage handlers are also potential thieves.
If you do not want to invest in permanent locks, then use plastic zip ties. The extra security deters thieves who may pick bags behind your back while you are walking.