Travel theft, whether it is a pickpocket in a crowded station or a hotel room search, targets predictable places: the wallet, the main compartment of a bag, sometimes a jacket pocket. The single most effective thing you can do is stop keeping all your cash in one of those predictable places. Split it, and hide the split portions well.
Wear it instead of carrying it
A money belt with a hidden zipped channel lets you wear emergency cash under your clothing rather than in a bag or pocket. This defeats pickpocketing entirely, since a thief working a crowd targets bags and pockets, not the underside of a belt. Keep your day-to-day spending money separate in a normal wallet, and treat the belt as untouched emergency funds only.
Use your own luggage against thieves
A hotel room search, whether by a dishonest staff member or someone who gets in during your absence, tends to check the same spots: the safe, the suitcase's main compartment, under the mattress, inside shoes. A false-bottomed water bottle or a stash can packed among your actual toiletries or snacks does not read as luggage worth searching, since it looks and weighs like what it claims to be.
Split across your body and your bags
Do not keep your entire cash reserve in one place, even a well-hidden one. A reasonable split is: a small amount in your regular wallet for daily spending, a larger reserve in a money belt or hidden pocket, and a final backup split into something like a hollowed lighter or a battery safe tucked in a different bag. If one location is found or lost, you have not lost everything.
Use the hotel safe, but not exclusively
Hotel room safes are a reasonable place for passports and larger sums, but they are not infallible, and staff or previous guests occasionally know default codes or backup keys. Treat the room safe as one layer, not the only layer, particularly for cash you would need immediately if something went wrong.
Avoid the obvious spots
Under the mattress, inside a shoe on top of the pile, in the first zippered pocket of a bag: these are the first things checked by anyone searching a hotel room or bag, professional or opportunistic. If you use physical hiding rather than a safe, choose something less obvious, like the bottom of a bag of laundry or inside a genuinely used toiletry item.
Think about airport security separately
Anything you carry through security screening should be something you are comfortable having examined, since metal detectors and X-ray screening will flag unusual dense objects. A metal money belt buckle, for instance, will need removing. Diversion items like a hollow battery or a stash can generally pass without issue, since they read as exactly what they appear to be on a scan, but do not rely on concealment to get anything through security that would otherwise be prohibited.
The core principle
Redundancy beats a single clever hiding place. Splitting your cash into two or three separate, unrelated locations, worn, packed and hidden in different ways, means a single theft, search or loss does not end your trip. Combine that with genuinely boring, unremarkable containers, and most opportunistic theft never finds anything worth taking.