Leave uninvited guests―your kids, your well-behaved dog, your work colleague―at home.
If you have a friend visiting from out of town, decline the invitation and explain why. The hostess might invite your guest, but you don't want to put her in an awkward position by asking if you can bring someone.
Don't offer to bring something and make her come up with an idea. Tell her you would be happy to make that appetizer she loves.
If she takes you up on your offer to bring something, don't forget the serving pieces―platter, spreader, cheese knife―that the dish requires.
Bring a hostess gift that's low-maintenance. Cocktail napkins or a bar of fancy soap is easy and doesn't need to be dealt with in any way.
Take pressure off the hostess by mingling with newcomers.
Pitch in when you notice things that need to be done, like refilling the ice bucket at the self-service bar at a friend's cocktail party.
If you're the newbie in a group, follow the crowd: If guests are in the kitchen serving, offer to help. But if the hostess is in the zone―simultaneously broiling, stirring, and tossing―don't upset her rhythm.