It's not just the words that you say that will affect your child but also the way you say it. That is paralanguage.
Is your pitch high or low, your pace fast or slow, your volume loud or soft and your tone friendly, serious, sarcastic or calming? These are some things that will affect the way your child perceive your words. You can say, "That's great!" in an encouraging tone or sarcastic tone and the meaning would be different.
When you feel frustrated or upset with your child, lower the pitch and volume - don't let it rise! When your child does a positive act then use your warm and enthusiatic voice. Take sarcasm out of your voice and always be encouraging even when your child does something wrong as you can encourage him or her to goodness.
Be concious of not only what you say to your child but how you say it as well.