I'm in an apartment too and I have to say that my biggest problem with Halloween decorating is that we have little to no extra storage ability which means that we can't invest in big, high-impact pieces -- it doesn't make sense to invest money in pieces that we have nowhere to put during the off-season, and I'm reluctant to spend the time (or money) crafting big pieces for the very same reason.
So everything 'big' that I make has to be either something I don't mind pitching or something that folds up and away incredibly easy. I use lots of cheesecloth and lots of cobwebs. We have a very art-filled apartment with lots of old gilt and wood frames, so switching out all of the artwork goes a long way.
For the third year in a row we're doing a haunted garret theme, emphasizing atmosphere over props. Every year we buy or I make a few new things, and throw in some new twists on the theme so it's not quite the same as last year. This year my room will be the coat room again and I'm thinking of doing something like this in the far corner to freak the guests out --
http://www.pinterest.com/pin/286260120040194253/
This is something that's great for our needs, because I can totally rig something up using black fabric and either a cheap mask, or maybe just a deep hood, with some sort of LED eyes.
I think the best part about throwing a party in an apartment is that, not only do people mingle really easily, but you can really decorate the entire place without completely breaking the bank. You can buy those printed wall scene setters and really go wild with them (especially if you have an apartment in a more modern building with lots of right angles and no architectural details, and want to disguise this).
I like to identify spots in the apartment where an unexpected scary element can lurk, like the laundry nook, or the weird nook between our kitchen and the bathroom. Or outside the bathroom window, for example. (I don't have pics, but I suspended a mask outside the bathroom window with a glowstick taped inside so it was a disembodied floating head staring in at you. People had a really love-hate relationship with that, haha)
We don't have a huge amount of space for food so going vertical however you can is really helpful (not that I took my own advice last year). Our teeny-tiny kitchen was crammed with the punch bowl and coolers, and I'd love to hear how other people include their tiny pass-through kitchen into their party decor, because so far I've mostly just ignored it, put up cobwebs, turned off the lights, and scattered as many candles around as I could.
I'm on the 2nd floor as well so we made sure to invite our downstairs neighbors, mostly as a head's up that it was going to happen, since it's two apartments in one building kind of deal. They didn't come but we also didn't get any noise complaints either!
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This got really long, sorry!