Catch everything! One of the resources in the game that is harder to come by is Stardust. You get stardust from catching Pokemon. Even if you already have 13 Ratattas and 42 Pidgeys still catch more.
Catch those commons. Some places get an obnoxious amount of Zubats, here I see Ratatta everywhere. Other people say they see too many Geodudes. When you get to level 9 you get a lucky egg and that is when you’ll evolve all your commons. If you evolve 8 pidgeys that’s 8,000xp. As far as I have seen it is the fastest way to level.
Capture gyms with at least 2 of your friends. Capture the gym and plan from the bottom up stick a 400 – 600cp pokemon in there then all three train against is. Rig the fights so you know everyone will win. Then when it’s level 4-6 put the other pokemon in the gym that are 500 – 1500cp.
Find a facebook group for your area where people are planning meetups. I’m going to downtown this weekend where several of my friends and maybe 50+ others will be. Incense + lures + meeting new people = winning.
Find a copilot or a driver. I drive my wife around and she catches Pokemon and she gets all the pokestops. I don’t even look at my phone. She just gets excited that we are both getting new creatures and all the goodies from the pokestops. It’s incredibly efficient. Just please, don’t drive and Pokemon. It’s way too distracting.