Wallets are mostly preferred to be carried in pockets. The front pockets or pockets on your jacket or coat are good options but unlike these, the back pocket which many people tend to use is not that great. Putting your wallet in your back means you would probably sit on it. That can leave damage on the wallet over time which can lead to deformation of the wallet. Also, sitting on a wallet causes misalignment of the spine and therefore bad for your health.
Besides the pockets, be careful with what kind of bag you choose to put your wallet in as plastic bags may cause the wallet to suffer from being moistured. That can damage the wallet and lower its longevity and make the overall maintenance more difficult.