Way back in my days as an aspiring hostess I made a lot of mistakes. By my sophomore year of high school, however (what can I say, I was an early bloomer with the party-planning) I developed a small yet highly accurate list of party killers - Light, Space, and TV.
Since that revelation in 1992, I can honestly say that the list still holds true. Of course, it does not apply to backyard barbeques, Guitar Hero tournaments, or Superbowl Sunday bashes. It's more for your average cocktail-type party. Here they are, broken down for you:
Too much light: Dark equals mystery. Mystery equals sexy. When it's dark, people feel less self-conscious and inhibited. I'm not saying it should be so dark that you can't make out the hors d'oeuvre you're about to eat, but it should be dark enough that you can't read the small print on a beer bottle. Use dimmer switches or replace bright bulbs with lower wattage. Experiment with a mix of dim bulbs, colored bulbs, and candlelight. A night or two before, do a lighting run through - you'll be glad you did.
Too much space. If people are spread out, they're not socializing. Again, it shouldn't be so cramped that you can't move, but you shouldn't be able to roll a beach ball down the center of the space, either. Having your guests in close quarters encourages conversation ("I couldn't help but overhearing you're also a big fan of Siouxsie & the Banshees, pre John McGeoch"). If you don't want the party spilling over into certain rooms, make them uninviting with a lack of seating, and zero light (especially no candlelight...partygoers are attracted to candlelight like moths to a flame).
Television. Obviously, the exception for this is any sort of viewing party. If you're not getting together with the implicit purpose of watching something, then there is NO REASON FOR TV TO BE ON. Let me repeat - TURN IT OFF. Nothing will kill a party faster. The main reason for having a party is for people to have fun and talk to each other. If the TV's on, you'll get a handful of wallflowers who will sit there glued to the tube, and from there it's a slippery slope into No Interaction Land.
Halloween bash, nary a party killer in sight
Of course, it's still up to you to provide good music, libations, and friends, but avoiding the Party Killers is the first step to throwing a memorable party!