The most important security feature in a backpack is to be boring. Buy a black backpack without fancy features or designer logos. The plainer the better.
The second most important security feature is that it hold everything of value and still be comfortable to wear.
This is how laptop theft typically happens to travelers abroad. "Watching" your laptop — or even using it — is no deterrent at all.
Gear up in such a way as to never have to leave your valuable electronics behind. Make sure everything fits into one backpack. (In my case, I carry a MacBook Pro, iPad, iPhone (my Android phone lives in my pocket), Google Glass, chargers, cables, extra credit cards and more. The total value of what I carry in my backpack is about $5,000.)
Then never, ever leave it behind. Always have your backpack with you.
When you're in a coffee shop, don't place the backpack on a chair or on the floor next to you. Put it on the floor between your legs and under the table.
Finally, whenever you're in a situation where anyone can be close to you and behind you, put the backpack on backwards so it's in the front, to prevent razor-blade access. The most likely way thieves will steal stuff is to divert your attention. If something happens that's attention-grabbing, always move the backpack to the front and keep your head on a swivel.
How to keep your laptop from being stolen
After surviving many trips abroad, working often in coffee shops, I laugh whenever I hear someone at Starbucks saying to a stranger: "Will you watch my stuff while I go to the bathroom?" This provides you with exactly zero protection against theft.
Here's the global best practice for stealing a laptop from an American innocent abroad: Grab it and run.
The commonly used defenses against this crime are all perfectly wrong. Sitting there won't deter them. They'll steal it even while you're actually typing. If you can out-run a teenager familiar with the surrounding streets (Good luck with that), he will drop it before you catch him, destroying the laptop. If you chain it to the table, he'll drop it and smash it trying to get away. If you chase after the guy with the laptop, his partner will casually walk off with your backpack.
To protect your laptop, follow my three-second rule: Make sure a crook can't run from your laptop to the door in less than three seconds. Find a place to work that's tucked way back in the corner with lots of tables and chairs between you and the door. And make sure a thief can't approach your laptop without you seeing them. It's more important that you keep an eye on who's approaching your table than your stuff.
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