Dress Like Snape
How are you supposed to put off that dark, evilish aura without the right clothes?
For starters, Snape wears a high-neck jacket with a lot of buttons. No one should be able to see your neck, or really any of your body. To compensate for that, his outfit it really tight. Now, we've really never seen what is under that jacket, but I imagine it's just a black button-up shirt. And then of course, the black underpants and dragon hide boots. And during potions class, he wears dragon hide gloves.
MOST IMPORTANTLY, he has a long, billowing cape. It has to extend to the floor, and still have a couple feet left to drag on the floor. There's nothing sexier than Snape sauntering down the corridors of Hogwarts, his cape billowing behind him.
If you don't own anything of this sort, it's fine if the best you can do is just wear all black. When you're a huge black spot in the middle of a vibrantly colored street(or hallway, whatever), people have no choice to to be slightly intimidated. Your demeanor should be enough to ruin everyone's day, but also attract a lot of people, because darkness is sexy.
But still, it's good to have a long, black cloak, at least. It shouldn't be that expensive. It's pretty much just a long jacket.
Either way, remember; black is everything.