Harry Potter and the Half-Blood Prince is probably the second darkest Harry Potter movie to come out (the absolute darkest being the next one, Deathly Hallows). Because of the misfortunes in Half-Blood Prince, be sure your party is set to the same mood: dark and chaotic.
Lord Voldemort terrorizes not only the wizarding world, but the muggle world as well. Capture this effect with these ideas:
Scatter toy snakes around the floor.
If you dare, draw and hang an image of the Dark Mark for your party space
Use Halloween decorations and hang Spider Webs around the party area.
Represent each House of Hogwarts with colored streamers:
Gryffindor: Red and Gold
Hufflepuff: Yellow and Black
Ravenclaw: blue and Bronze
Slythern: Green and Silver
Turn on only one or two sources of light in your party space, leaving the rest of the room illuminated by white twinkle lights and candles.
If you're akin to Gryffindor, create a big poster of the Fat Lady and hang it on the wall.
Put broomsticks in the corner of the room. Label them as "Firebolts" or "Nimbus 2001"