First and foremost, arrive (at least) 30 minutes early for the ceremony. The time stated on the invitation is the time that the bride will be walking down the aisle. Do not be the guy that has to wait for the entire processional to be done before you even take a seat!
(Once the processional begins, be quiet!)
Secondly, please make sure your phone is on silent... or turned off... Or better yet, leave it in your car! Once again, you don't want to be "that guy" whose phone starts blaring Marvin Gay during the middle of the wedding vows.
Speaking of cell phones, no photos during the ceremony! This is not a concert and there's a reason they hired the professionals.
And for the love... do not wear white. Seriously, no one in attendance should have on a white dress except the bride. It's as simple as that!
Speaking of attire, please keep it classy. We all have that crazy aunt and/or friend who shows up dressed a little too scantily clad. Do not let that be you! Remember, this is not your day and you are not supposed to be the center of attention. The bride is!
If you are attending the reception, be sure you have attended the wedding first.
Be there when the couple cuts the cake and has their first dance!
Do not talk negatively about the bride, the groom or their taste in decor. Remember, it is not your day! It's their day and their wedding reflects the two of them as a couple.
Do not complain about the drink or food options that are available. Once again, the bride and groom chose the food and beverages that they like. Simply enjoy! They are free to you, after all.
Do not over drink. I know it can be a challenge sometimes when there is an open bar, but you do not want to be "that girl" doing the worm in a dress or "that guy" who is still doing keg stands behind the bar.
If the couple has a #weddinghashtag they are using to document the fun had by all of their guests, please make sure to keep your photos appropriate. No one needs to see how (insert name here) had a little too much fun and is now spending the night next to the toilet. Those are not the photos they are looking for!
Don't pick up the microphone. It rarely goes well.
And last, but most certainly not least, absolutely NO divorce bets should be made while in attendance!!!