The happiness of your pokemon really doesn't do too much unless you're under one of these conditions:
1. Your Pokemon knows return or frustration
2. Your Pokemon evolves from friendship

Really that's about it. Yup, pretty useless but I'm gonna write about it anyways.

**How to raise Happiness**

1. Walking. Yes, that's right. For every 256 steps that you walk, your Pokemon gains one point of happiness. If you attach a soothe bell to the Pokemon, it will gain 2 instead. A good way to raise happiness without using any items or money is to attach soothe bell and bicycle around cycling road or through Solaceon.
2. Give it poffins. The kinds that it prefers give it more happiness (Like Modest Pokemon like Dry Poffins)
3.Vitamins. Each Vitamin gives 10 EV points and also some happiness. If you're already going to EV-train you may as well put 100 in the stats you want to make your job easier.
4.Give it EV reducing Happiness gaining berries like Tomato, Qualot, Pomeg, etc.
Once you have got your pokemon fully... happy(?), just level it up to see if it evolves.

**How to lower happiness**
Well unless you hate your pokeymanz there's not much point in this. Return is better than Frustration in most cases as it is much easier to keep them happy than to keep them really unhappy.
But if you really want to, just let it faint or give it bitter medicines like EnergyPowder, Energy Root, and Revival Herb. Thats about all i know though >=\