1. Be happy for the bride and groom or don't show up
I once went to a wedding where the bride's aunt, a nasty woman whom practically no one liked, simply showed up after not being invited, and sat at one of the main tables reading a book during the entire reception, like wtf. If you hate the couple getting married, if you're bitter about your divorce/life/that you're "still" not married, if you're pissy about "having to spend" loads of money on a plane ticket, hello! don't go to the wedding. You were invited for a reason, and that is to celebrate the couples marriage and love. If you can't or don't want to be happy for them or are just going to be a bitter Betty, skip the wedding and do something that is going to make you happy rather than you ruin their day with your ugly comments and actions.
2. Abide by the dress code
Guess what, today is NOT about you. If it were, your name would be on the front of the invitation. We know you live in jeans and sneakers, but it's a formal wedding not a backyard ho down. Rent a tux, thrift a formal gown, borrow a dress from a friend, do what you gotta do, but this is not your day, your way, it's the bride and grooms. You don't get to just decide you're going to do whatever because you think it's stupid or unfair. This actually may result in you not being allowed into the festivities if you can't comply for a scant 4-5 hours for the ceremony.
3. Drugs and Alcohol abuse are an absolute no-no
Getting drunk, or getting messed up on some drugs for a wedding should not be something on your days agenda. Show some respect to the families, to their friends, and to yourself. If you have a drug or alcohol problem, I would suggest getting help for that and not using a wedding as an open invitation to display your addiction for all to see and/or potentially ruin the wedding with your antics.
4. Don't invite people to someone elses wedding
The invite said Mr. Smith, not Mr. Smith plus one, not Mr. Smith and family, not invite 5 other people because you don't want to go it alone. If you absolutely want to invite someone and they are not expressly invited in your invitation do not just bring them or at the very least, ask the bride and groom if it's okay, because usually there are assigned seats at the reception and if your table of 8 suddenly has 12 at it, there will be someone who knows who is or isn't supposed to be there. Also don't just bring your kids. Again, if the invite did not include them or the words, "and family," you need a babysitter because they are not invited. When it doubt, ask first.
5. Don't interrupt the pre-show
The bride and groom and parties involved are super busy before the ceremony getting ready and doing what they need to do and having some very personal and private family and/or religious moments prior to the ceremony. Do not take it upon yourself to burst in a changing area or start taking up the bride or grooms precious time by just deciding you're going to pay them a visit and talk their ear off. Save it for later.
6. Don't interrupt the ceremony
If you are late, tough luck, wait outside or wait for an usher to allow you in. There is noting more distracting then being in the middle of the vows, and you and your family are clamoring in loudly or trying hard to be quiet, but you drop something and all eyes turn. Show up at least 15 minutes before the start of the ceremony or wait until its over if the bride and groom have already walked down the aisle.
7. Don't comparison shop
Don't be that guest who points out how "at my wedding, I had a live band" and express how you think "DJ's are always tacky." Guess what, you've had your day, and this is theirs. The bride and groom did not select things you would personally like and approve of. They selected things that are personal, or that they like, or most importantly, can afford. It's not your place to sit and say how bad or so not something you'd ever do at your wedding, their wedding is. If you must, save it for after the wedding when you're in your car on the way out vs. saying what's on your mind to other guests who may be very close to the bride and groom or have helped with the wedding planning themselves.
8. Don't show up with your surprise dietary restriction
Trust me when I say, it is way too late to request a vegetarian meal for the BBQ reception...at the BBQ reception. Read your invite. If it asks you what you want to eat, you better go ahead and select something as per the bride and grooms requests or risk being stuck with something you can't eat, or worse, that you are allergic to, and for God sakes, if you have life threatening allergies or food intolerances, even if it's not on the invite, make for certain you let the bride/groom know far in advance and don't just assume out of their 100 guests and a million things to do, that they are specifically thinking about you and your needs or should have remembered.
9. Don't upstage the bride
For the ladies, a lot of wedding traditions are long gone by the wayside, but this one remains. The bride is the only one that should be wearing white/ivory. Now is not the day to wear a long flowy ivory dress, or show up in some big blingy number that is incredibly eye catching. Also this is not the time to wear your skimpiest or most see through clothing. Keep it together. You can go back to "being you" after their ceremony. Oh, and unless you absolutely have the permission of the bride and groom, do not take this opportunity to propose to your own girlfriend at someone elses wedding. Rude.
10. Don't become the photographer
The bride and groom have often shelled out thousands of dollars for professional pictures. If you fancy yourself an amateur photographer, today is not the day to prove to everyone what a supposedly good shot you are. Stay out of the photographers way. Don't jump in front of their shots, don't follow them around or suggest that you'd like to get some shots like this or that or that they should shoot the bride/groom a certain way. The photographers are professionals that don't work for you, they work for the bride and groom, so let them do their job.
11. Don't give long or awful roasting speeches
No one needs to hear the entire anthology of how you met the bride and groom in your speech, nor do the mom/dad/bride want to hear the nasty details of the grooms former hook up habits in college. This is not the time to ugly roast the bride and groom and embarrass them in front of everyone they know. And for God's sakes, if you can't get through a speech without long winded ugly crying, keep it short and sweet and get out of there. We get it, you love them so much it hurts, but hurt later...on your own time.
12. And finally, don't make their day about you
This is one of the absolute lowest things you can do. This is not the day to argue with the bride or groom, or bring up some fight you had 10 years ago that you want to reignite, or start a fist fight with the best man, or launch into some speech about how it should be you on the alter, or whine about the bridesmaids dress, or how long the ceremony is going to be. Shut up. It's only a few hours. Leave your problems at home and let the bride/groom have their day.