Stick To Training A Balanced PartyThere are really only two ways of training your Pokemon in Pokemon GO, just like in the Mainstream Games, either by committing all of your time, Stardust and Candy into one Pokemon or in a balanced distribution among a number of your Pokemon. Both ways have pros and cons to them, but the far more effective way is committing to a balanced team.
When using a balanced team, there are plenty of more advantages than using just one Pokemon. First off, a more balanced team will often mean you are evolving more Pokemon, which, in turn, means you're furthering your Pokedex (the main concept of the gaming series). A more marginal team also means you'll usually have six or more equally great Pokemon to take on Pokemon Gyms with. And when some gyms have seven, eight, nine, or even ten Pokemon in all, it helps to not lose much power from one Pokemon to the next on your team. You'll also have plenty of Pokemon to leave at a number of Gyms, which means you'll easily rake in more PokeCoins for defending them.
However, there are a few setbacks for this way of training. For one, you'll probably never have a Pokemon that will be as high as some Pokemon you see left at Gyms by the Leader of such. The process of raising a number of Pokemon also means it will take more time for you to be good enough to take on Gyms with the persistence of winning. It could even take more of your resources, such as both Candy and Stardust, to do this form of training compared to raising one single Pokemon, at least if you're doing it right anyways.
Slow and steady usually wins the race, and that's why this is not doubt the more effective way with going about Pokemon GO. Raising one Pokemon might get you the greatest Pokemon of Pokemon GO, but it won't give you the constant success as a balanced team will one day get you.