In general, PEA's and amphetamines are pretty damn solid. Nonetheless, standard precautions include keeping them away from heat, moisture, light, and pets/children.
I store mine in little amber vials with tight-fitting caps. Everything goes into a Sentry safe that is locked and kept in a cool, dry place. In summer, the whole thing goes in the fridge next to the broccoli.
To be on the safe side, I wouldn't carry your DO-x around in the wallet if it's going to be in your back pocket for long periods of time. The heat and moisture may affect it. Probably not, but it's possible.