We've hosted several parties at our house, now, and though we're by no means experts, people seem to have a good time, so I thought I'd share what we've learned. (note: learning it doesn't mean we've got the execution down, but I think we're at least headed in the right direction.)
1. Invite everyone you know. (Only half of them will show up anyway.) Presumably you have cool friends, right? And they have you in common, right? Good enough. Inviting only a couple crowds will result in divisions and cliques - if everyone only knows a few people, things seem to mix more.
2. Buy more than you think you'll need (but only of things you'll use later). Why yes, I do have two big jugs of rum and a 1.5L bottle of champagne left over from New Year's, but it's not going to go to waste.
3. Don't fight the crowd. A fairly large group is sort of its own entity. People are going to gather in places you won't necessarily anticipate, and they have a tendency to entertain themselves. Best to let them go where they want to rather than risk losing momentum.
4. Clean thoroughly, but not to the point that it's going to break your heart to have to do it again 24 hrs later. No use scrubbing the grout before people come over.
4b. Accept beforehand that someone is going to spill and/or break something. It will probably be whatever is most likely to stain, on the worst possible place, in the most expensive glassware you have available. Accept this, and when it happens, you can cheerfully hand the guilty party the broom they're asking for.
4c. Don't invite people who wouldn't ask for a broom after shattering glass all over your floor.
4d. Don't have any closets/areas/rooms you'd be ashamed to have people see. Inevitably, someone will see it. It's more welcoming and much less stressful to be able to show people all around your house anyway.
5. Ask people to bring something. Pick either drinks or food, or let guests pick between the two and see what happens. People like having something to contribute. (Unless of course you're rich enough that you can afford to drop several hundred dollars on one evening, in which case, these suggestions are probably useless to you.) The other benefit to this is that, assuming most people bring something, you'll have enough no matter how many people end up attending.
6. Find something simple to decorate with that you can stick in all corners of the house. For the Pirate New Year's, it was predominately hemp rope (which we got two spools of and wrapped around everything) and tapered candles stuffed into old wine bottles. For the last Fourth of July, I got a couple of bouquets of white flowers and put them into every clear glass vase I had.
7. People are probably going to get in drunken debates about religion; as the host, try to avoid getting involved. We've yet to accomplish this one, but it's a good plan.
8. If people are going to be intoxicated, have a pile of pillows and blankets ready. At some point in the night, people are going to be intoxicated enough that they'll spend a few hours passed out somewhere. If you have a pillow to hand them, they'll be very grateful, and you'll feel better about letting someone sleep on your floor.
9. Themes can be fun, but make them simple. If whatever you pick is too complex, it'll be difficult to get people to participate.
10. Enjoy yourself. Parties take their color from the host. If you have fun, other people can't help but have fun, too.