The words you speak program and condition your mind. When you speak words of gratitude, you are programming and conditioning your brain and mind to experience more and more gratitude.
Speaking words of negativity and ingratitude program and condition your brain and mind to experience less and less gratitude and more and more misery.
You create your habits and your habits create you. By speaking words of gratitude regularly, you are developing the habit of speaking words of gratitude. The more you keep up this habit, the easier it becomes to say even more words of gratitude. As you continue to speak words of gratitude, thoughts of gratitude are integrated into your mind and you will spontaneously think and speak this way.
Catch yourself whenever what you say is an expression of a lack of gratitude. Instead of being upset with yourself for still lacking as much gratitude as you would wish, be grateful that you are becoming more aware of your lapses. This awareness will enable you to be more careful from now on.
It would be a good idea, to resolve that whenever you hear yourself saying something that is ungrateful, you immediately make five statements of gratitude.
(From Rabbi Zelig Pliskin's book: THANK YOU! Gratitude: Formulas, Stories, and Insights: Artscroll Publishers)