Make use of things you already have on, such as socks and underwear. Slip some bills into your sock or put your cash in a zip-top plastic bag and slip it into the front of your undergarments.
Wear a money belt with a hidden zippered compartment in the inside of the belt. Do not wear a money belt with a visible pouch as that is an advertisement to potential pickpockets that you're keeping your money there.
Keep your money or wallet in a front pocket rather than a back pocket. If someone goes into your front pocket, you'll feel it more quickly than if your money was in a back pocket.
Sew a hidden fabric pocket with a Velcro closure into your pant leg prior to traveling. Keep your money hidden in the pocket.
Cut a small vertical slit in the waistband of a pair of pants. Keep folded bills hidden in the slit.
Split up your money and other valuables between various pockets or between two different wallets. If you are mugged, the thief will most likely only get a little bit of your money.
Wear cargo pants and keep your money or wallet in a fastened pocket. A pickpocket will have to take longer and have a harder time accessing your cash. This will give you time to realize what is happening.
