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)
I haven't been able to test this out on my own Eevee yet (because I can't find one aaargh) but based on the number of independent confirmations it seems legitimate.