The practitioner is admonished to speak the truth or, more accurately, to restrain from speech that is not true.
What helped us most was simply practice. Lots of practice.
Right speech is therefore intentional speech that rejects mindless chatter, gossip, slander, and lies. When we are honest with ourselves, self-reflection often reveals that much of our speech is harmful at worst and unnecessary at best.
My words reflect my thoughts, my thoughts reflect my beliefs, and my beliefs, especially the unexamined ones, run my world.