Force your Eevee to evolve into the Pokemon you want. Either Vaporeon, Jolteon or Flareon.
To do this, you just rename your Eevee to name of the trainer from the TV Show.
Rename to Rainer for Vaporeon
Rename to Sparky for Jolteon
Rename to Pyro for Flareon
There are rumors that this only happens for the first time you do it for each one, and also that in the future, you will be able to do the same for Gen 2 Eevee evolutions, Espeon and Umbreon. Another rumor is that it will be dependent on time of day as well.