Ruiz means by "be impeccable with your word": don't have negative thoughts about yourself. Basically don't be hard on yourself, rather you should have a good self image.

That appears to be a small part of his meaning, based on this quote copied from Wikipedia: "Be Impeccable With Your Word. Speak with integrity. Say only what you mean. Avoid using the word to speak against yourself or to gossip about others. Use the power of your word in the direction of truth and love..."