View yourself as being a person who is grateful and fervently wants to keep upgrading his level of gratitude. This will lead to more words and acts of gratitude. This is who you are and it is an integral part of what you do.
"But what if I'm not really a grateful person?" some ask. Right now, make a verbal or written list of ten things you are grateful for.
Once you do this, you are a person who has expressed gratitude. Therefore you have a right to begin to view yourself as someone who is already on the pathway to being a grateful person.
Every grateful statement and action will strengthen your self-image of being a grateful person. This will lead to even more words and actions of gratitude and your positive self-image in this area will become increasingly solidified.
(From Rabbi Zelig Pliskin's book: THANK YOU! Gratitude: Formulas, Stories, and Insights: Artscroll Publishers)