Give it a soothe bell, feed it lots of vitamins that increase EV's like Protein, Zinc etc.
Also walk a lot with your pokemon, try doing some breeding and cycling around, stuff that
gets you walking a lot, each step will increase the hapiness. As far as I know, Pokethlon does not
increase happiness. Also, sending a pokemon into battle increases happiness as well, but don't let
eevee faint, as that decreases the hapiness. So make sure you know it's something you can beat
or a pokemon you can definatly run away from. Hope this helps!