Pokemon GO may be a game aimed at casual players, but that sure won't stop any self-respecting Pokemon trainer from power leveling. The best way to do that is using a combination of either Incenses, Lures and a Lucky Egg, an item which doubles the amount of experience you receive for half an hour. Unfortunately, all of these items are in rather short supply and can only be reliably acquired by spending real money in the in-game shop and redeeming those coins. However, some players have recently realized a way to basically acquire an unlimited amount of Incense and Lucky Eggs - or more accurately - a way to make Incense and Lucky Eggs you already have last forever.
The hack itself is rather simple. First of all, activate the Incense and Lucky Egg from your inventory then exit the game and proceed to your device's settings menu. From there go to the General tab and tap on the Time and Date settings. Turn off automatic time and date option and manually set your smartphone's clock back by 30 minutes or less. After you get back to the game, you should see that the time left on your Incense and Lucky Egg charges has increased by however many minutes you've moved your device's clock. Note that the game's code prevents Incense and Lucky Eggs from lasting more than a full hour, so never set your phone's clock back more than 30 minutes because it either won't do anything or will completely deplete your Incense and Lucky Egg charges, depending on some yet unspecified circumstances.
Note that some players are reporting that this hack doesn't work on Lucky Eggs, or at least that it doesn't increase the duration of their effect despite prolonging their timers. However, the cheat seems to be working perfectly as far as Incense is concerned. Do keep in mind that this practice is most probably considered cheating and might get you banned by Niantic, so proceed at your own risk. Of course, there are also plenty of legit methods of catching rare Pokemon, so it's recommended players try them out first before deciding to risk everything and game the system.