What to find out in advance: Ask how people will be dressing. If you have tentative plans with a couple of friends, it is OK to ask if you can bring them to the party, says event planner Kimberly Schlegel. But if you want to bring four or more people, think about throwing your own.
What to give:

A morning-after basket of teas and coffee, muffins or scones, and some aspirin.

A desk calendar and a selection of cards for birthdays, anniversaries, and other happy occasions in the new year.

What to do:

The best way to show your appreciation for being invited is to get into the spirit of the party, says Schlegel. If it’s a masquerade, wear a mask. If the host wants everyone to wear sparkly “2005” glasses, swallow your pride.

How to dress: In cold, dry winter weather, elegant fabrics such as silk are prone to static cling. To maintain a flowing look, run a wire hanger over your clothes to remove the electric charge.
Coming and going: Above all, avoid arriving early. For a cocktail party, your host probably expects you no later than an hour after the start time. If you’re not sure when to leave, Schlegel suggests looking for these signs: music turned down or up (to make talking more difficult); lights turned up; bar closed; food no longer being replenished; the host with her shoes off, rubbing her feet.
How to follow up: If you took pictures, tuck a set in with your thank-you note. (Or e-mail digital photos, but send a handwritten note, too.)