What is the best way to get stronger Pokemon in Pokemon Go?

Now that I am at a higher level, I find Pokemon that are also at higher levels.  For example, when I was a lower-level character I had a Flareon and I invested a ton of Eevee candies/stardust into that Flareon, making it as strong as possible—about 800 cp.  But then two days later, after I power leveled and gained 5 levels, I caught an Eevee that was around 550 cp, evolved it, and without powering it up at all I got a 1200 cp Vaporeon. 

Don't make the same mistake I did.  Wait until you are a higher level before using your candies and dust to level up your Pokemon.