I've been a Maid and Matron of Honor.
1. Shop for bridesmaid dresses, tell her everything she picks out is fine and you will be HAPPY to wear it, even if this entails a bow on your butt that is bigger than your head! I'll never forget my SIL's wedding all the maids were complaining about the dress she picked out for us to wear making her feel bad. When she left the room I said "Hey didn't she wear all the dresses you picked out for your weddings, now you tell her you like it or ELSE!"
2. Organize the wedding shower, you should get financial help from all the maids. It doesn't have to be a bank account buster. I also had all the maids pitch in for one nice shower gift.
3. Remind her about stuff like matching panty hose for the dresses, same color nail polish, same necklaces, shoes, etc. I offered to buy the hose for everyone, and had all sizes and several packages in case of runs. You can return what you don't use. Remember stuff like the good luck penny in the shoe, something old, something new. My MOH made my garter belt which I thought was really nice.
4. Plan a girl's night out to coincide with the bachelor party night.
5. Have a good sense of humor, keep her calm and relaxed on the wedding day.
6. Come up with tasteful, heartfelt slightly humorous wedding toast.
7. Don't tell her the groom is a big fat loser and go haha I told you so when they get divorced 10 years later.
If you are still best friend's after all this then Congratulations!