How can I force my Eevee to evolve into the Pokemon I want?

The Eevee Brothers from the anime were named Rainer, Pyro, Sparky and they owned a Vaporeon, Flareon and Jolteon respectively.

If you nickname your Eevee either Rainer, Pyro or Sparky, your Eevee will evolve into that desired Eeveelution!

Sparky = Jolteon

Rainer = Vaporeon

Pyro = Flareon

You can also see it here:

Please be aware that you can do it only once per Pokemon class (to reword, you can only do this trick once per nickname).

Normally Eevee will evolve into a Random version of the 3. However an easter egg has been spotted which allows you to choose which version you want to evolve it in.

Simply change the name of your Eevee to one of the names of the trainers in the original TV series and you will get the corresponding Eevee evolution.

Sparky - Becomes a Jolteon (Electric)
Rainer - Becomes a Vaporeon (Water)
Pyro - Becomes a Flareon (Fire)

Most people consider Vaporeon to be the strongest as it has the highest CP, but it kind of depends on what kind of Pokemon are in the Gyms around you.