This concept of the Eight-Fold Path of Buddhism basically sets the foundation for the proper way of living. One thing I really like about the idea is that the way to a good life is not set in stone. It isn't like the Ten Commandments and is open to interpretation. Some critical thinking is useful here in understanding how to best apply the concepts to your own life, because everything is not black and white; there are shades of gray when it comes to ethics, morality, and goodness.
When I think about what is the right thing to say I often think of the old adage, "If you have nothing good to say, then don't say anything at all." I don't always believe this statement, as sometimes we have to be critical of things we can't stand for, sometimes we have to defend others in the face of discrimination or ridicule. Right speech doesn't always mean we have to be positive, it means that we have to say the best thing to lead to a good life. Sometimes this means not being a push-over.
More than this saying, I like to follow my favorite piece of advice from the Tao Te Ching.
Those who know, do not speak.; those who speak, do not know.