Inviting 30 people over to your house for beer pong is probably a good reason to bring your ancient Roman vase collection to the attic. Anything you don't want touched, messed with, or potentially broken needs to put out of sight. Shut the doors to rooms you don't want entered, lock them, tidy up the bathroom, and get your home in party condition.  You'll probably want to have garbage bags and cleaning supplies at the ready, too, just to make everything easier. Have games and alternative choices of music out just in case the party needs a little revamping. Get a couple of your closest friends together to help prepare your house for the party. This is also a plus for when guests start showing up; it looks like people are already there and in case someone not so well-known shows up early, it'll save you from an awkward situation. Direct each guest as they show up to the designated food, beverage, and entertainment areas. You want them feeling comfortable right off the bat. Mingle and get to know the people you might not know so well. If there's alcohol involved at your party, make sure everyone who wants to drink has a drink in their hand. Turn the music up enough so that you can still comfortably hear one another. Make sure your guests are comfortable with one another by introducing those that aren't known that well to your group of friends. You may also want to start with the party activities and turn the music up louder to enforce a party atmosphere. Monitor the alcohol counter to keep anyone from overdoing it with hard liquor. For groups that don't know each other, you may need to start conversations rolling. That's why you're the host!  If the party starts reaching a lull, feel free to call it a night. Just start cleaning up and telling people thanks for coming and they'll get the hint. If they don't, just tell them the party's over! They don't have to go home, but they can't stay there. Make sure everyone gets home okay. Do they have your number? Are they okay to drive? Does anyone need a ride? If they can't drive, do you have a sofa or bed they can crash on? If your entire living room is a pyramid of aluminum cans and delicately placed paper plates, you have more than enough right to ask your guests to help clean up. It's their mess, too! If that thought makes you nervous, ask a few of your closest friends to tackle the beast before they leave. You'll pay them back when they host next time!

Summary:
Prep your home, if applicable. Get everything set up. Keep your guests happy. Ask others for help cleaning up.