One of my concerns when I backpack is not to give the impression I'm a rich tourist (which I'm not anyway...), in order to not be scamed nor be the victim of a pickpocket :greenguy: , and in more poor countries to avoid being harassed and show them respect. So the kind of day pack we use may bring undesired attention to us.
What a friend of mine does is carry around his day stuff in a plastic bag from a local shop of the country he's travelling in (grocery store for example).
I use an across the shoulder day pack that looks warn out (I've made it so by brushing it hard and putting it in the drying machine a few times), and I've never had any problems, even in places that are said to be risky for tourists, like Bucharest.
Oh, and even though you probably know it, never ever carry all your valuables (passport, visas, etc.) in your day pack.
No magic will prevent you from loss or thieves. I've met a few people who had serious problems because of that... The "stomach pocket" is the best I've found.