1) HOOKAH. I cannot emphasize to you how many times they got me laid in college, like, at least a couple dozen directly traceable back to this. Have a nice, tall hookah with a good light tobacco (peach), and instant-light charcoal. Spend $100 on socialsmoke.com and I promise it will pay off.
2) Chill lighting. When people are lounging around, have a switch (or better yet, get a $20 remote) and be able to turn on a bunch of black lights or some LED color changing light strips. Really helps set a chill mood. If nothing else, just have some miscellaneous dim lights that are not the overheads.
3) Nice bottles of shit. As much as everyone thinks they can tell what's what, most alcohol tasting is psychological (the New York Times did an amazing piece on this relative to wine tastings using fMRI imagery). Get ahold of a few bottles of nice scotch, vodka, rum, etc, then keep washing them out and refilling them with cheap shit. In 5 years of college and being in a fraternity I never got caught and had a reputation among everyone as a classy guy who always had the finest shit. (Fun fact: According to the NYT study, stuff being in nice bottles doesn't just trick people into
thinking they enjoy it more... it actually changes how their brains process the taste data and leads to an quantitative increase in enjoyment. So you are in fact being altruistic and doing everyone a favor by tricking them. Yes, that's how I justify deceiving my friends. It was for their own good!)
4) Seating. Depending on how big your room is, a futon, some bean bags, extra chairs, couch, etc. always helps. If you can get 10 people chilling comfortably then you are in good shape. A lot of people just have a bed, and people don't like chilling on beds (in beds, however, is another story).
5) Retro video games. I had an SNES and an N64 and bitches be crazy for Mario Kart 64 (hurry though, soon college kids won't know what MK64 was). There are some great Mario Kart drinking games like finish a beer during the race, or do a shot every time you lose a balloon during battle, etc. Guitar Hero is also good sometimes but maybe getting too old-school.
6) Phone chargers. Have a generic micro and mini USB phone charger, as well as a spare iPhone charger that you keep in your room. Nobody carries theirs around, and if you end up having girls spend the night spontaneously, they'll appreciate being able to charge up. Plus you don't want them tripping out and worrying about having a dead phone, which many do.
7) Personal items. Even if you don't wear them, have some contact lens solution, decent shampoo, and makeup remover and crap like that hidden somewhere. Every once in a while a girl would be over and start having eye problems, or want to spend the night but worried about her makeup getting stale or whatever the fuck, so saying "oh, someone left this makeup remover over here once, here you go" helps, which leads me to:

Other girls' shit. I was always finding hair ties, clips, pins, and other random shit I have no idea what to do with around my room. Rather than toss that shit out, leave it near your bed or in another location that it is likely to be seen by other people. It quietly says to people that you have girls around and is a nice, subtle DHV.
9) A decent screen. Don't waste money on an HD TV... spend $350 on a refurbished brand-name projector on Ebay, buy a $70 projector screen on Amazon, and wire your shit up. Most good TVs now cost more than a decent projector does, and again, people will know you as the guy with the awesome media setup.
etc.
<B><I>"Whether you think you can or think you can't, you're right.</I> - Henry Ford</B>