Here's the golden rule of being an awesome bridesmaid: If you don't have anything nice to say, don't say it at all.
That means no trashing the groom or other bridesmaids, friends or family. This rule is particularly important when wedding dress-shopping with the bride. If she's nuts about a gown and you abhor it, keep your critical thoughts to yourself and find something about it that you do like. Give positive feedback.
And don't fake it; your friend can probably tell if you're being authentic and honest. This rule applies to everything: her wedding colors, the cake flavor, the song played for the couple's first dance. Remember, the bride is putting her heart into planning this momentous event that will kick off a brand new chapter in her life. Her fingerprint is on everything, so any carping from you could easily feel like a disparaging remark about her personally.