According to this post on the /r/pokemongo subreddit, you can force your Eevee to evolve a certain way by changing its nickname before evolving it to:
- Pyro to evolve into Flareon.
- Sparky to evolve into Jolteon.
- Rainer to evolve into Vaporeon.
(These are the names of Eevee Brothers from the TV series/Manga)
You can also, as of the Generation II update, use nicknames to evolve into a certain Eeveelution, but like before, this works only once.
- Tamao to evolve into Umbreon.
- Sakura to evolve into Espeon.
This now works with the Generation IV evolutions Leafeon and Glaceon. When renaming Eevee the first time, the evolve button with display the evolution it will evolve into rather than a question mark.
- Linnea to evolve into Leafeon
- Rea to evolve into Glaceon
This technique will only work once per evolution. For example, if you have already evolved an Eevee named Rainer into a Vaporeon, evolving another Eevee named Rainer will result in a random evolution.
Evolving without the naming tricks will give you Flareon, Jolteon or Vaporeon by random chance.
Also, according to another reddit thread, by walking 10km with Eevee as a buddy you will get Espeon when evolving during the day and Umbreon when evolving during the night.