Here's some tricks I use to help my Sim pull off an epic party.
*Decorate the space! Don't underestimate the importance of environment! Buy balloons and banners, change the lighting scheme. If you want, have a firework or two on hand to launch just to keep things alive and interesting.
**Day and time of day is important to the type of party your Sim is throwing. My birthday parties start in the early afternoon (1pm or 2pm). My house parties will start closer to when the sun is setting. Corporate parties (such as the company Christmas parties - Not gift giving parties) will always be held on Saturday in the early evening.
My Tricks:
1). Weather permitting, hold the party outdoors either at the host's house or at the park. If indoor parties are more your thing, in Sunset Valley the Art Museum is a great place to hold an indoor party.
2). Have plenty of food available as Sims are greedy (at least the ones in my game are). I usually make two main dishes and one dessert type. Be certain to decorate the table the food is placed on with a simple centerpiece.
3) Have the music pumping (turned to high volume) and your Sim dancing before the first guest arrives. As guests arrive, click on them and select 'ask to join to dance' (or whatever the option is). For some strange reason, when a sim asks another to dance, the fun meter for the one who asked begins to raise instead of both of them. Also, the louder the music is, the further Sims can hear it.
4) After the guests have been dancing for a bit, turn the radio off just long enough for the Sims to register that the music has stopped. Once they all stop dancing, turn the music back on and leave them to their autonomous actions. Sims will start to eat, socialize, engage in an activity or choose to keep dancing all on their own. Be aware, sometimes fights break out or Sims pass out!
5) I only add a mixologist if the party is being hosted by a top celebrity as it keeps with the expectations of the lifestyle. Otherwise, pop in a regular juice bar or a juice keg (if you have University Life). But watch out for any party animals who visit your party as they will 'spike the punch' if you know what I mean ;)!
6) As @IreneSwift said, having things for the guest to engage with will go a long way in keeping the party alive and Sims from yawning from boredom. I add the gyroscopic whatchamacallit doohickey thingy to the lot and the Sims seem to really like the 'rush' they get from it.
Other than that, just sit back and watch them do their thing. You, your Sims and their guests will have a blast! Also, sometimes there's a Sim or two who don't know when the party is over, when you've had enough just kick them out by asking them to leave. Wake up any of them who attempt to camp out on your lot by falling asleep on the lounge chairs or sofas. Some sims seem to want breakfast the next day and will try to stay on your lot.
