One of the main things you hope your guests will do at your wedding is have fun. But fun doesn't include causing a total raucus on your special day. If you're a bride and groom unsure of appropriate behavior, or a guest at an upcoming affair, take note of this bad behavior and avoid making a wedding-guest faux pas of your own!
During the ceremony, the most important thing to remember is to be respectful. This is not the time to chat and catch up with old friends, or be on your phones. Pay attention and be quiet — there will be plenty of time during the reception to have a good time! And if it's a religious ceremony, participate in as much of the service as you feel comfortable.
For the reception you're obviously allowed to have a good time and let loose, but since this a special occasion you shouldn't go overboard. Don't grab the microphone to sing a slurred song, or incessantly clink your glass. You also shouldn't monopolize the bride and groom's time in the receiving line. Obviously you want to all celebrate together, but the bride and groom have to give their time to everyone — have your catch-up another time, over dinner when they return from their honeymoon, not during the reception itself.
After the reception is over, the décor is not a free-for-all. This means you shouldn't be grabbing for a centerpiece to take home unless the bride and groom tell you too — the same goes for food and any other flowers sitting around.