I got asked to be a bridesmaid recently and I could not be more excited. The wedding is in Boston next August, so we're going to try to build in a vacation in New England and possibly a road trip as well. Plus, there are four bridesmaids and three of them are friends of mine from college, and they are super-easy-going, and the MOH seems really nice too.
So I'm trying to follow the rules of bridesmaiding that I have come up with, through several years of reading blogs, acting as the emergency person, being a bridesmaid, and getting married myself and having an awesome crew:
1.) Be involved and be available. If possible, make time to go dress shopping and squeal over magazines, or at least share a pinterest board. It helps the bride not feel like a bridezilla that is the only one that cares about looking at fluffy dresses.
2.) Reign the bride in. Sometimes it is your job to say, bluntly, "do not cook all of the food for your wedding yourself", sometimes it is your job to say, "you are taking on too many DIY projects" and sometimes it is your job to say, "I'm not wearing four inch heels, that is insane."
3.) Don't make her wedding about you. It's fine for you to obsess over your hair and makeup, and you should discuss it with the bride a bit so that she knows that you're excited, but don't spend the day of the wedding acting like your hair is a priority, or pouting because you hate it. It will make the bride feel upset that she asked you to get your hair done.
4.) Don't wait too long to order your dress or your shoes (if the bride is particular about shoes). Don't order your dress three sizes too small just because you are planning on losing weight. (If you are heavily dieting, consider trying to order dresses from a retail store that lets you do returns, but you might have to suck it up and pay for alterations.)
5.) Don't gripe about looking fat. It will upset the bride, because she thinks you are beautiful and she doesn't want you to feel upset or out of place. If you want to lose weight, lose weight, but don't make a huge deal about it or make the bride feel like you did it because she wanted you to. (Note: it is acceptable to politely mention that certain lines of bridesmaids dresses do not carry sizes large enough for you. This is not something that many people who have skinny privilege think about. It is also acceptable to politely request to shop at a place that carries dresses in multiple sizes, so that you are not squeezing a size 16 person into a size 8 dress.)
6.) Don't give her much input about the bachelorette party. She is allowed to request a particular weekend or a particular activity. Bachelorette parties are a big thing that leaves the bride feeling sad and upset - often turnout is low, people bail at the last minute, or nothing turns out exactly as planned. Better to not know what is planned.
7.) Do NOT complain about the dresses. Especially not after they come in. Complain to somebody that doesn't know the bride or isn't going to go to the wedding, and then put on a big smile and tell the bride that you love it. (I loved my bridesmaids' dresses. My bridesmaids' did not. They all totally faked their enthusiasm when I squealed about them, and for that I am forever grateful.)
8.) Put your makeup on as quickly as possible on the day of the wedding, so you aren't still getting ready when everybody else is ready to go. Do it in an area away from the bride, so you aren't crowding her and you are all at the same sink. Bring a mirror for you and/or the other bridesmaids. Test run your makeup beforehand and don't ask everybody annoying questions about whether they think a particular lipstick looks good.
9.) Get your dress altered. One wedding I emergency crewed for, two of the bridesmaids were standing in front of the mirror complaining about how their dresses didn't look right. One had lost a lot of weight, but not gotten the dress altered. It looked terrible. I shortened her straps for her and ten minutes later, the whole thing looked a lot better. This is not me tooting my own horn - it took ten minutes, it would not have cost much at an alterations place, and she should have gotten it taken care of. The other one was fixed up with some artful safety pinning, but she would have been much better off if she had gotten it altered.
10.) Have fun and be nice! Have a good time. Even if you're not a dancer, get out there on the floor and have fun. Even if you hate stupid games at showers, play them and get into them. Make friends with her family members and act open and inviting towards everyone - don't act jealous or weird of the other bridesmaids or of her family or close friends that aren't in the bridal party. For example, in Bridesmaids, while I think the Helen character was kind of out-of-line in the way she acted, Kristen Wigg got super-possessive of her friend, which was a total turnoff and I wound up hoping that the bride would make new friends soon.
Those are my rules (more like guidelines, really) - what are yours?