I was going to do something better than this, but I have places to be and things to do, so I picked one that wouldn't require much time. Sorry.
As we all know, Snape's hair is extremely greasy. Some say it's because he doesn't take showers, but I say he takes plenty of showers, and it's because he just has naturally greasy hair that he can't help. Although, not taking showers would be a good way to get hair like him.
I have naturally greasy hair, which won't go away no matter how many showers I take. My parents make fun of me and constantly tell me it's gross, but I really don't mind. In fact, I consider myself quite lucky to have hair like Snape.
Well, it's not exactly like Snape's, because it isn't straight and it's not black. But at least I have one aspect right.
So to get it greasy enough, either don't take showers that often, or use hair creams that make your hair not frizzy and straight. Or, if you're lucky like me, just move onto the next step.
Snape has pure black hair, which is really rare. Most people will say they have black hair when they really just have dark brown hair. So you should probably dye it.
His hair is a bit past shoulder-length in the first movie, but you can tell he lets it grow out throughout the series. In the Half-Blood Prince, it's actually really long. So your hair has to be long, but not too long. I suggest letting it grow out to a few inches past your shoulders, but stopping it there.
You should also straighten it, or use a cream (like Chi) that'll keep it straight and greasy at the same time. However, allow you can tell there's a couple curls in Snape's hair sometimes, so it doesn't have to be completely straight.
Also, he parts his hair in the middle, so your hair should hang evenly on the sides.
Here's a pic so you'll have a good idea of what your hair should look like:
