Elope.
But seriously - if you receive an invitation, respond in writing within one week.
Mail a gift, if you send one; using their registration list is thoughtful, saves on returns.
Don't bring another guest unless the invitation specifies one.
If you attend, arrive about 10 minutes early.
If you don't know what to wear, phone the person who invited you and ask.
Sit wherever the usher directs you.
Stand, sit, or kneel along with everyone else.
Don't receive communion unless you share the religion of the couple.
Shake hands with everyone who makes eye contact.
Don't get drunk.
Don't smoke indoors.
RESPOND TO THE INVITATION IN WRITING, IMMEDIATELY.
Tip the bartender, if there is one.
Chat politely with old people.
Don't criticise anyone.
I must add: DO NOT wear white, even if it is a June wedding and your choices are limited. It's the bride's day. She gets to look like the cake.