Tell yourself this: Talking to yourself can improve your mood. Or ruin it.

What you say to yourself is very powerful, researchers have discovered. If you're feeling down, you can give yourself a pep talk that will perk you up. But beware, because the opposite also is true: If you're in a good mood, you can talk yourself out of it.

Many scientists believe that every single thought we think triggers a chemical reaction in the body. Good thoughts produce good chemistry. Bad thoughts result in feeling weird and unhappy.

Your moods can affect your choices, lifestyle and relationships.

For instance, people judge the success of your relationship with them based on how you make them feel, psychologists report. So it's natural that others tend to enjoy your company more if your moods are reasonably harmonious and upbeat.

This is especially true within families. If you're feeling anxiety or stress, it's easy to let your guard down at home and vent to your spouse and kids. They pick up on your negativity, and soon everybody's in a bad mood.

Don't let bad thoughts ruin your day — or your family's. These tips can help:

• Practice speaking positive sentences to yourself. Say silently, "I can do well in this business meeting today. I will try my best to contribute a few good ideas." Don't say, "This meeting will be a disaster."

• Write down your internal dialogue. Find a notebook to document your shaky thoughts and fears. Go back over the list later to review what thoughts might need some practical fine-tuning.

• Instead of grumbling under your breath about problems, reflect on good things in your life.

• Don't be too harsh on yourself. People make mistakes. We forget to pick up milk at the grocery store. We misplace our car keys just as we're getting ready to leave for work. Vow to improve without belittling yourself.

• Feed yourself powerful thoughts whenever you face a challenge. Tell yourself, "I can do this."

• Periodically take a break to monitor your mood. What are you feeling? Why? And is that feeling justified?

• Think of positive words as a tool that can move your world in the right direction.

Judi Light Hopson, Emma H. Hopson and Ted Hagen contributed to this report.