you have it ALWAYS in the team, never making him faint, giving her pokeblocks and that stuff.. don't give him the items you can buy in lavaridge town! Y'know, the bitter root and that stuff, 'cause it taists awful. If you're playing in johto, go to the subway and go to the stylist. If you're in kanto (new version), go to blue's sister and make her to clean the pokemon up.
If you're in sinnoh, go to the amity square.
thats all. In no time he will be evolving.
but remember! eevee evolves in espeon in DAY