Etiquette rules that still apply
Etiquette, in all forms, often has a negative ring to it. The mere mention is enough to drum up bad memories of snarky old ladies primly commenting on ladylike and gentleman-like behavior -- when they weren’t simply harrumphing and moving along. In times past, the rules of proper etiquette literally filled a book and governed everything from RSPVing to bringing a dish to allowable dinner conversation.
Fortunately, these days we’re a lot more easy-going.
Still, that’s not to say that good manners and etiquette have gone out the window entirely. Some rules of traditional etiquette still apply, there just aren't as many as there used to be. This means, though, that the rules that have survived need to be observed even more carefully, lest you been seen as ill-mannered.
These days few rules dictate what you should and should not wear to a party. However, two broad guidelines apply when choosing a wardrobe. First, if the party has a theme, dress accordingly. It’s impolite to show up to a 1930s party in jeans and a T-shirt.
Second, unless you are following the first rule, don’t wear anything that would make you (not the party itself) the center of attention. What this means will vary from party to party, but at least consider wearing what you would wear to work to a party.
Bringing food and wine
More and more parties are being thrown with the allowance for or expectation that guests will bring an item. Of course, it is up to each individual host to determine whether they want their guests to bring something. Still, if you think your host would be fine with you bringing a dish, here are some simple guidelines.
First, avoid bringing wine you expect to be served with an entrée. Unless you have specifically spoken to the host about pairing a wine with the main course, your host probably has that covered. If not, there’s a good chance your wine and the main course might not pair well. Instead, opt to bring a dessert wine or something that goes with light appetizers -- or choose a more unusual drink like a lambic or hard cider.
Second, if you want to bring food, bring something that is temperature-stable and easily sharable. Dips are great for this, as are small sandwiches, kebabs, meatballs on a toothpick, etc. Desserts should also be sharable, though because they are served at the end of the meal, they don't necessarily need to be temperature-stable.
Do RSVP. It’s rude to show up without telling your host that you will attend. If too many unexpected guests attend the party, the poor host may not have enough food.
Also, let your host know if you are going to bring guests -- and do not bring more guests than the number you RSVP'd. Anything else will put the host in a bind and is just rude.
Sending thank-you cards is largely a lost art in modern times, but it never hurts to follow up a party with a handwritten note. Pictures on Facebook are nice, but a real "thank you" mailed via the postal system will mean so much more.