Wearing white
Don't do it under any circumstances. White is reserved for the bride and it is her day.

Bringing guests
If the invitation does not say "and guest" or "and family" the person the invitation was addressed to is the only one invited.

The wedding gift
Don't bring it to the wedding, mail it to the bride and/or her parents in advance, even if it is a check. Gifts can get lost at the ceremony and lugging it around is no fun either. Gifts should be sent before the wedding takes place. The whole thing about waiting a year after the wedding is hooey. If the wedding doesn't happen, they will return your gifts.

The ceremony
Attending the marriage ceremony is not optional. The whole point of a wedding is to watch the actual event take place.

Cameras
Take pictures at the beginning and at the end of the ceremony, but no where in between. They hired a photographer for a reason.

The reception
If you RSVPed to the celebration, you should remain at the party at least until the wedding cake has been cut. After they smoosh each other in the face with cake goodness, you can leave if you really have a bug up your butt.

Good-byes
Wish the wedding couple well, congratulate the bride & grooms families and thank the people who paid for the whole shebang. Then and only then can you leave.

This information was collected from Mannersmith