How do I maximize XP gain?

I don't have an exhaustive list, but I do have one tip that would be very efficient.

Lucky Eggs are items that double the XP you earn for 30 minutes. The ideal time to use these is when you have prepped yourself to maximize your XP gain. Some great conditions to use an egg would be when:

• You'll be staying at a Pokestop and have Lures and Incense to use
• You have a lot of Pokemon ready to evolve. Evolving Pokemon provides quite a bit of XP.
• Getting Nice/Great/Excellent throws. Each of these require you to throw the Pokeball within the targeting circle. Nice throws happen when the circle is large, while excellent throws happen when the circle is pretty small. They add a bonus of 10, 50, and 100 XP on a successful capture respectfully.

So far, I've gotten Lucky Eggs from leveling up. You can also purchase them from the shop. If you hold off on evolving and get comfortable at a Pokestop, you could gain quite a bit of XP in a short amount of time with the use of a Lucky Egg.

The single fastest way to get XP is to save up tons of level 1 evolution of your local pokemon that evolve with just 12 candies.

you get 500 exp for every evolution, so if you save up 12 pidgeys and have 144 candies, and then pop a lucky egg you will make 12,000 experience in just a few minutes.

So save your evolution's to use with lucky eggs.