Get that wallet out of your back pocket!
That's what I tell almost every guy that walks through the door. Over time, sitting on a big wallet can lead to muscle imbalance and back pain. (see photo of what your spine looks like while sitting on a wallet).
It's like sitting on a phone book, but under only one buttock for years on end. Over time, this can cause a hip hike or rise on one side and not the other.
Research has shown that people with chronic low back pain typically have the pain on the side that they keep the wallet on. Sometimes the position of the wallet can even push on the sciatic nerve, creating pain down the leg.