I'm giving this advice as a former bride, a former bridesmaid, AND as a wedding photographer. Not all of my tips will apply to everyone, not everyone will agree with my advice. But there are some common themes that I hope will be helpful to everyone considering becoming a member of a wedding party.
Being a member of the wedding party is a big responsibility. It's signing on for months of activities, dress fittings, traveling, parties, expenses. It is time consuming, it generally costs quite a bit of money, and it's also an honor to be asked. The bride and groom are asking you to stand beside them on one of the happiest days of their lives, bearing witness to their love, their commitment, and pledging your friendship & support as they start their journey as a married couple.
As a wedding photographer, I see all kinds of wedding parties, from intimate groups of just a best man and maid of honor to a grand party of 30. I really enjoy taking wedding party pics...it's a chance to show off not only the hair & make up, gowns & tuxes that took forever for the group to decide upon, but a way to express the kind of couple you're representing. But it's also one of the biggest challenges I face as a photographer, especially when someone (or someones) aren't really into it.
Perhaps you accepted the duty because you felt obligated. Maybe the bride or groom was in YOUR wedding party. Maybe they're a family member. Maybe they asked you in front of a room full of people and you said yes because you didn't want to say no or because you didn't have a chance to think it through. On behalf of brides & grooms and yes, wedding photographers everywhere, when you're asked to join a wedding party, give it some thought. Make sure you have the time in your schedule, the money in your bank account, and the joy in your heart.
Be responsive to emails/texts/phone calls. It doesn't matter if they're asking you to pick out a pair of shoes or give them your size or if you'd prefer crab cakes or chicken. The more timely your response, the easier you're making things on the bride & groom. I know it's not as important as responding to say, your boss, but maybe you could pretend that each email question has a deadline.
And on the big day...be helpful. Ask the bride & groom how you can help them best. Maybe it's letting the groom crash in your spare bedroom the night before. Maybe it's giving grandma and grandpa a special ride to the reception. Maybe it's keeping an interfering relative away from the bride. Maybe it's being in charge of the gifts or assisting the DJ in identifying key people. There are a gazillion and one tasks to be done and many times, the bride & groom don't want to ask their wedding party peeps to do more, feeling that they've obligated them enough. But seriously...you've come this far. You've spent the money and thrown the bridal shower or the bachelor party. You've been to ten dress fittings. You're wearing the most uncomfortable shoes you've ever worn, a ridiculous vest, or a dress that cost you $500 that will never see the light of day again. So ask how you can help. It will be appreciated more than you could ever know.
Remember...smile. Smile a LOT. The camera sees more than you realize. Believe it or not, I can tell if you're jealous of the bride, if you think the groom isn't good enough for her, if you hate your tuxedo, if you think the bride's dress makes her look like a Vegas showgirl or an oompa-loompa. Most of all, I can tell if you would rather be getting a root canal than standing beside your cousin/sister/bestie from third grade/coworker you barely know. IMHO, if you really don't want to be there, you probably shouldn't accept the offer...but if there was no way around it (it's hard to say no to family especially), consider it the acting role of a lifetime. :)
Be wary of the open bar. I know it's tempting to drink everything in sight and then dance the night away, but you want to make sure that you are still "on call". Maybe you're a designated driver, or you're keeping an eye on the kids under 12, or you promised to direct the out of town guests back to the hotel. It's totally ok to kick up your heels and truly celebrate your friends and the special night, but do it responsibly.
As far as the formal photos go, usually taken right after the ceremony, trust me-I know it's hot, you're tired, and you really just want to get to that open bar and stuff some appetizers in your face and then exchange those stilettos for some flip flops. But make the effort to enjoy the photo time. Stay nearby, tell your friends & dates you came with to enjoy the cocktail hour while you do your photo duty, and don't leave the immediate area until you're officially dismissed. All in all, it's about 20 minutes of formal portraits. If everyone listens carefully and does what the photographer asks, it will go that much more smoothly and quickly, everyone will look great, and you AND your bride & groom will have a very special keepsake to grace the walls of the house for years to come. And hey, if we get through all the formal posed ones, we'll have time for some funny ones, creative ones, jump shots, model shots...you name it, we can make it happen. But it takes dedication and cooperation. If you have suggestions for fun poses, pull the photog aside privately and make your request. I personally LOVE ideas and try my best to accommodate all requests. If you feel like some specific pose is lame, remember...chances are the bride or groom made the request for that specific shot beforehand. When it's your wedding, you can call the shots on posing...until then, just try to smile and enjoy it as best you can, however cheesy it may feel. :)
I hope I haven't scared anyone off of their lifelong dream to be a bridesmaid or a groomsman. I've always been so honored to be in the weddings of dear friends and family. If you don't know what that honor will entail...ASK. Ask the bride or groom. Google proper wedding party etiquette. That will help you make an informed decision and really be the best kind of friend to have in one's wedding party.
P.S. Having wedding party members who help out the photographers?
Bonus. At my most recent wedding, I had the lovely Tara regularly asking how she could help us, pointing out who people were, where things were located, answering our random questions, and even offering to get us food & drink. It rocked. :)