Follow the Bride's Lead: If she's asking you to pick whatever you like, or to browse the racks and help her get inspired, ask for a little guidance first. Does she want everyone in chiffon, or is she open to multiple fabrics? Is there a length or detail she'd prefer? Don't grab a mini-dress if the event will be black tie, but if she wants some new ideas, feel free to look toward complementary colors or a silhouette she may not have considered.
Give Constructive Feedback: Wearing a dress that you totally hate? Keep the strong feelings to yourself. Instead, find a helpful way to vocalize what you don't like, whether it's a shade that's not working on your skin tone or a neckline that's way too revealing for your liking. If you can explain what you don't love, you might either be able to find a way to fix the situation with alterations or helpfully guide the bride in a new direction.
Wear the Right Undergarments: If you know you're heading to a bridal salon to look for bridesmaids' dresses, make sure you're wearing nude underwear and tuck a strapless bra into your purse. This way you'll be able to try on any color or neckline with relative ease, and won't be distracted by bright red boyshorts or a lacy bralette that sticks out in all the wrong places.