1. DON'T leave guests to awkwardly mingle at a party you're hosting. "If it's a crowd that doesn't know each other, it's important to give people something to go on," says Lizzie Post, etiquette expert and great-great-granddaughter of manners maven Emily Post. When making introductions, try to jump-start the conversation—explain where you know each guest from, or bring up something they have in common, whether it's the same alma mater or just an undying love for Celine Dion.
2. DON'T get drunk if you're the hostess. "Hosting is a responsibility," cautions Post. "Know your own limits." Use caution when you're the guest a party too, and slow down once you feel yourself rapidly approaching that point where you think strip beer pong is a good idea. When you feel like one too many people has asked you, "are you OK?"—it might be time to put down the wine and grab a glass of water.
3. DON'T be a Debbie Downer when giving a toast. "Short and sweet" is key, according to Post. Stay away from anything that mocks the host and puts down the time of year or occasion being celebrated.
4. DON'T bring an uninvited plus-one to a party unless you've cleared it with the hostess. This includes boyfriends, children, and pets. DO bring a dish or a bottle of wine instead.
5. DON'T apologize profusely if someone gets you a gift and you're empty handed—or worse, lie and say your gift for them got lost in the mail/is being custom-made overseas/flew out of your car window on the drive over. "Just say thank you and leave it at that," advises Post.
6. DON'T forget to match the food you serve to the attire on your invitation if you're hosting this year. "If you invite people to a black-tie," Post says, "I would consider not having spaghetti at dinner."
7. DON'T forget to ask about your guests' food preferences and dietary restrictions—and try to make at least one dish they can eat. Not sure how to make vegan brisket? "Ask!" says Post. "Find out how to make something from your guest." The bonus: "the majority of people will almost always offer to bring a dish." If you're the guest, whether you're keeping Kosher, vegetarian, or South Beach, bring a dish or two and let the hostess know your situation beforehand—when you RSVP, not the morning of the party.
8. DON'T let the party drag on if you're hosting, and DON'T be the guest that overstays her welcome by four hours. "Don't walk up to your guests and take their drinks from their hands," Post cautions—if you're hosting, give guests the hint by cleaning up the bar and putting away food. Pay attention to those same hints if you're the guest. If you're still chatting away over a freshly-poured martini while your host has changed into pajamas, it's time to go.
9. DON'T be a doormat when it comes to having friends or family stay at your place over the holidays. "Stand up for your own sanity," Post advises. If you can't fit your in-laws and their two Saint Bernards in your studio apartment, say so. The same goes for guests that have overstayed their welcome. "The fish and the house guests go bad after three days," according to Post. Make it clear before their arrival that you would love to host them between certain dates, and point them to a hotel for the duration of their stay.
10. DON'T cry over spilled wine. Stop fretting over whether or not your napkins match the tablecloth, and don't sob uncontrollably in the bathroom if you fumble your toast. Just enjoy the company and the festivities, and be glad that holiday stress comes only once a year!