Going by how my uncle has thrown parties I've found the following to be a good recipe for a party:
1. Music, DJ or played through a computer on several speakers is always good.
2. Adding to that, karoke is always fun.
3. Alcohol is a no brainer, its always more fun if you make cocktails. My uncle made this thing that tasted like coffee at first, but once you'd drunk a couple of them you'd be fucked. Like drinking for several hours fucked.
4. Food is also a good idea to soak up alcohol and whatnot, also for hungry people and fat shits like myself.
5. Consoles can be fun for some people, perhaps set an Xbox or Playstation up with 4 pads if possible and a game for people to rotate on. Whenever I go to my uncles for a party someone starts a CoD or Fifa tournament which almost everyone ends up taking part in.
6. Finally, silly drunken antics are always lulzy. The one year my uncle came out wearing glasses and holding a mop covered in bleach between his legs. We were about to get in a taxi to go home, and my uncle being utterly smashed screamed he was Harry Potter, before seeing our taxi driver was black, shouting something about a cleaning spell and ramming the mop in his face. Obviously you'll want something less racist, but stupid drunken antics/games are always fun.

I reckon trying too hard is actually the worst thing, let is be spontaneous

Shit. Where I live all you need for a good party is a lot of people, and a lot of alcohol. It helps a lot when you have a pool (and/or hot tub) and a beer pong table. Unless you mean a different kind of party, then I don't know what to tell you.

keg :D

Oh, and be responsible. Alcohol may be a no brainer, but that shit can still cause problems, so try to limit the use of it.

But that's entirely up to you, of course.

Play drinking games.
Or wait till everybody is drunk and then play kid's games.

Or you hire some escort ladies.