Number One: No White.
Unless you a part of the wedding party and the Bride specifically puts you in white, as a guest, you should under no circumstances wear the color white because it is explicitly and traditionally reserved for the bride. Even if the bride is wearing an nontraditional colored wedding dress, it would still be more respectful of you to not wear white. There are the exceptions of your dress being patterned where white is not the main color, but you just want to be careful. It would put a damper on the special day if your wardrobe choice caused the bride -who is already stressed out- to send you angry eye daggers for matching her on her big day. This fashion don't may seem obvious, but it is surprisingly ignored or forgotten quite often. So, be the best guest you can be, and don't rain on your bride's white and lace parade.
Number Two: Don't dress for the club.
Every guest wants to look their best, and for some, their best means looking smoking hot. We just suggest that you tone it down for the wedding. Avoid mini skirts, extremely low cut tops, cut-out dresses, and anything overtly flashy or head-to-toe sequinned. Even if the wedding reception is designed to have dancing and more of a party atmosphere, any wedding event asks for a more modest fashion choice in order to not 'steal the show'. This does not, of course, mean that you can't have a little spunk in your dress, just that your best party dress may be one step out of place at a wedding event.
Number Three: Don't wear anything you are uncomfortable in.
Weddings involve a lot of time, even for the guest. You may be walking from the ceremony site to the reception site farther down, the parking may not be as close as you thought, or you may end up dancing -or the at the very least standing- most of the night. Whatever it is, you don't want to be caught in uncomfortable heels that you have never worn before. This "don't" could very easily be translated to "Don't wear new heels". It may be tempting to splurge on a new pair of heels that seem to start calling your name as soon as that invitation gets put up on the fridge, so if you do buy new pumps, break them in and don't save them for the wedding. Also, remember that whatever you wear, you are stuck in it. So avoid materials that itch and don't try and squeeze into anything that is too tight. It might look killer in the mirror, but if you are going to be scratching at yourself or fidgeting in it, then it might turn into a flop rather looking hot.