Do...
RSVP before the deadline.
We understand that you might have a lot going on this summer. After an entire year of limited-to-no social gatherings, we are really making up for it. As soon as you know that you don't have a date conflict, get that RSVP back in the mail!
Show up early with time to spare.
This doesn't mean showing up an entire hour before the ceremony starts. You should plan to arrive at least 20 minutes prior to the start of the event to give yourself enough time to find your seat and get settled in. No one likes the interruption of guests funneling in late...especially not the bride and groom.
Take your wedding favor.
If the bride and groom provide a wedding favor to you as a guest, take it! A lot of time, effort and money go into wedding favors. Whether you like it or hate it, make sure you take it with you when you leave.
Sign the guest book.
Trying to think of something meaningful or witty to say on the guest book can be a lot of unnecessary pressure. Guests often skip signing for that reason. Make sure you take time to sign the book, even if you just sign your name.
Don't...
Assume you can bring a plus one.
Unless your wedding invitation states "and guest," do not just assume you can bring a date.
Ignore the dress code.
Unless the wedding invitations state something different, pick an outfit that would be considered business casual. Ladies, if you are attending an outdoor wedding, wear flats instead of heels. And whatever you do, don't wear white or a shade that is close to white unless you want to hear faint whispers and see pointing from those around you.
Complain...about anything.
No wedding day is perfect, unless it is your own. You may encounter some things that are less than ideal. Perhaps you have to sit through a ceremony in a church without air conditioning or get a salty appetizer. Keep your complaints to yourself...at least until you get home!
Over drink.
Open bars can make for a pretty enjoyable evening. They are great places to try cocktails you've never had before. Not only are they on the house, you have the entire evening to sample. If you are planning on partaking, make sure you have a full meal or some appetizers beforehand. Another great rule of thumb is to treat yourself to a glass of water between each cocktail or glass of wine.