1. Abide by the invitation.
If the invite allows you a plus one, awesome. Don't add a plus one. If the invitation states that there aren't going to be children at the wedding, don't bring along your toddler that just loves throwing things. If you have a question about the invitation, ask!
2. Think ahead when it comes to your outfit.
Whether you are the maid of honor, best man, or the second cousin that was invited because you had to be, make sure you know exactly what you are going to wear and make preparations accordingly. If you're in the wedding party, then you practically have everything picked out for you already. If you're a guest, make sure you know what the dress code is (how fancy or how casual?). However, there are other parts of your attire that you should pay attention to despite if your outfit is picked out for you or not. Make sure to dress comfortably, or bring a back up (pants for a skirt, extra pair of stockings, comfy shoes, etc). Don't wear brand new things to a wedding. You're going to drive yourself insane with discomfort. Break in your new shoes, bra, shirt, tie, etc. For the gentleman and cowboys out there, know how to tie a tie, and for the country girls bring your comfiest boots--throw a little Bling on them!
3. There's a wedding registry for a reason, so try to stick to it.
I know you might be trying to get rid of that waffle maker you got two Christmases ago, but if it's not the one on the couple's registry, wait until this Christmas to give it to someone. Building a wedding registry is actually pretty tough despite what you may think. There's a strategy to it, research, time, etc. So, check the list and see what you have to work with. If there are items in there that you feel like you cannot access, afford, etc then figure out what kind of service you can provide in lieu of a gift on the list instead of showing up with that waffle maker. Some couples even prefer people helping out with wedding details. Be sure to ask!
4. Be social, and leave the petty stuff at home.
Weddings are usually pretty large parties. Whether you know everyone or no one, remember names, be social and don't be afraid to get to know people. This allows other guests that are also unfamiliar have the opportunity to get comfortable and have a nice time as well. No matter what social stigma might be going down, be sure not to add to the drama. Weddings are already stressful enough. Have fun and forget the feud.
5. Know your limits, and don't push them.
Whether your weakness happens to be alcohol, food, dancing, etc make sure you know your limits. Don't get so drunk that you can't function, don't stuff your face until you throw up, and please don't try that new break dancing move you saw on YouTube.