Paul Petrov of the Ballet Russe, backstage at the JC Williamson New Year's Eve party, Theatre Royal, Sydney, 31 December 1936 / Sam Hood, originally uploaded by State Library of New South Wales collection.
1) If some of the party will take place outside, make sure you use mosquito coils and have mosquito repellant available for the mosquito prone.
2) Supply enough chairs for two thirds / half of the guests. You don't want to have chairs for everybody, there'll be too many chairs and there'll be less "party flow". But some people will want to do some sitting down at some point. That's why I think 2/3 or 1/2 is the optimal ratio. If you are throwing a really big party though, people don't expect chairs as much. I am only talking about 30 or so people.
3) There should be no fluorescent lighting. Everyone looks terrible under fluoro and it makes you feel like you're in a meat fridge or office. Warm/soft/non-direct lighting only!!! For any lightbulbs, 60W or less and not direct! (ie, no ceiling lights, just lamps pointed upward, downward or at a wall).
4) You should have a stock of "emergency drinks" on hand. Ie, a carton of beer for the inevitable occurrence that someone will forget or run out of drinks. Add to that, some dry ginger ale, tonic water and coke, for people to mix with. Add to that, a cheap bottle of spirits.
5) People need to eat snacks if they are drinking. Due to the costs of providing emergency drinks and so-forth, at our last party I asked people to bring their own snacks. I think this worked well and nobody minded.
6) Make sure the party starts at 8pm or later. I think this is the unofficial code for: "we will not be feeding you dinner". For instance, if a party starts at 6pm then I think the host needs to feed food to the guests or at least assist in food facilitation ie, with a barbeque.
7) Have some blankets on hand for when people get cold.
8) Ask your most music obsessed friend/s to DJ/supply the music.
9) Make sure there are no dirty dishes in the sink, people might need to use the sink for preparing their drinks.
10) If you are the host, spend more time with the people you see less frequently and/or the people who don't know many other people at the party. The friends you who see regularly, you'll see them again soon, plus they know your house and can look after themself more easily.
11) Have some ridiculous decorations/something that people can play with. People enjoy novelty items.
12) Take lots of silly photos so you can more easily remember what happened.
13) Put away your valuables/things that could be broken easily.
14) Use lighting to direct flow of people. If there are any rooms/parts of the house that you don't want people to go onto, make sure all the lights in those areas are OFF. People won't really hang out in blackness.
15) Make sure there is an ample supply of toilet paper, very easily accessible in the toilet so there is no possibility of it running out, or people not being able to find it.
16) Don't go to bed until the last person leaves.
17) If someone has passed somewhere, make sure they are breathing/in a position where breathing would not be obstructed by surprise vomit.
So there you have it, Karen's guide to throwing a party. I think if I have to wrap it up in a nutshell, make sure people are physically comfortable, drinks are flowing, and there are no lights shining in their face.