Learned from a friend, probably people already know. This works when you have Lucky Eggs. You will get two at level 10, or you can purchase from the store. Using one Lucky Egg can gain twice experience, and it lasts for 30min.
Preparation is to get lots and lots of Pidgey candies, and Pidgeys. Evolving each require 12 candies, and you gain 1k experience each. So if you have, say, 480 candies, you can evolve a total of 40 Pidgeys, and you get 40k experience. To fully utilize the 30min double experience time, imagine each evolving takes roughly 30sec, then theoretically we can get 60k experience for each egg. Of course, that requires you to prepare 720 candies, which is hard but possible (equivalent to catching 240 Pidgey/Pidgeotto/Pidgeot).