Keep all your pidgeys you catch! When you get to a higher level you get awarded a "lucky egg". It gives you boosted XP for 30 mins. Pidgeys only cost 12 Pidgey candy to evolve. Them, Weedles, and Caterpies are the cheapest to evolve.
Hit lucky egg > Evolve tons of pidgeys > get 1000 xp for each evolution > power level! > Profit!