Keep your wallet up front. It's easier to feel someone trying to take something out of your front pocket than your back one. Some wallets are even curved to more easily fit into a front pocket.
Attach your wallet to your body. If you're carrying a purse, wear it with the strap going across your body. A chain that attaches your wallet to your belt loop can also help.
Leave fancy jewelry and name-brand clothes at home. If you look rich, a pickpocket is more likely to target you.
Stay alert when someone distracts you. It's common for pickpockets to work together, with one distracting you by asking for directions or causing a minor accident, while the other takes your wallet. If someone tries to distract you, pay more attention to what's going on around you.
Look down. Adults sometimes train small children to be a pickpocket--it's easier for them because of their small hands and innocent faces. Children may also work in groups to distract you.