Walking
Walking will raise the happiness level of your Pokemon. Walking through large areas for long periods of time give the Pokemon attention. This attention translates into happiness by the step. Between 35,000 and 38,000 steps will begin to raise the happiness level of that Pokemon. Cycling Road and Fuego Ironworks are among the best locations to do large amounts of walking.
Battle
Another way to raise the happiness level of your Pokemon is battling. A lot of battling will raise the happiness level. Always keep the Pokemon whose happiness level you are trying to raise in your party. The more battles that the Pokemon is involved in, the more experience it will gain. That experience will translate to happiness.
Massages
Massages also affect the happiness level of the Pokemon. Use your map to travel northeast to the Veil City. You will find a young girl in the middle of the city giving away free massages. Give the Pokemon you want to level up a massage. The only downside is that you can only give a Pokemon a massage once per day.
Food
Food makes any Pokemon happy, but you have to make sure it is the right food for that Pokemon. Check the Pokedex to gain information about the Pokemon whose happiness you are trying to raise. Its favorite foods will make its happiness rise. Be careful about what you give each Pokemon. If it is not the Pokemon's favorite food, then its happiness level will decrease.