What are all the ways to increase Friendship in Pokemon X/Y?

Solution 1:

Boosts Happiness

• Walking 128 steps
• Getting a massage (Cyllage City)
• Using vitamins
• Levelling up
• Using EV berries
• Soothe Bell (Shalour City)

Reduces happiness

• Fainting
• Using bitter medicine

Trading resets a Pokémon to its base happiness

Here is the happiness page from Bulbapedia. It has not yet been updated for XY, but the boosts from Gen V have not altered.

Solution 2:

You can usually get a Pokémon to have it's friendship raised in many ways really. Like, if you give your Pokémon vitamins like protein, carbos, etc.. You can also raise it's friendship by letting it hold a soothe bell, walking around with it or just plain battling with it! Then if you level it up it might evolve! But beware! PokemonAmie does not raise your friendship, but it does raise chances of critical hits, evasion, and status moves such as endure. And stay away from letting it faint and feeding it herbs, hope I helped!

Solution 3:

You could also use the Befriending O-Power with soothe bell on your pokemon then ride around the prism tower in lumoise city to gain friendship fast