Cellphone and wallet go in left front pocket. Keys, pen, and any loose change go in the right front pocket.
This leaves me easy access to whatever I may need in the most number of situations where I'll need it. Keys in the right pocket so I can grab them with my right hand in order to start the car (ignition on the right). Pen and loose change are the same deal, though I group them with the keys because I like to avoid scratching the ol' iPhone screen. Loose metal is havoc on a screen, and mine's still in great shape after nearly 2 years for this very reason.
Then there's the practicality of putting the wallet on the left with the phone. Since these two items are bulky and of similar size, having them with the keys would present an annoying inability to quickly reach for keys in times of need. Keeping the wallet and phone on the left also leave them available for easy-access whilst driving, thanks to the lack of interference from the seatbelt buckle on my three-point safety belt.
The back pockets are useless, save for receipts I've yet to throw away or articles of paper I care little about. Not to mention, I have an adversity to being pick-pocketed that leaving the back pockets empty helps to relieve.
My system is the most superior. Not a day goes by that I am not carrying wallet, keys, and phone. In fact, it's rare that you'll find those items anywhere but within the confines of the pants I'm currently wearing, or those which I was wearing previously.