Something that makes you proud. It can be anything like "I am proud of the person I am becoming." Or it can be "every day I am getting closer to my ideal body weight." Or "I am proud of my career choices which led to the salary I now earn ". The sentences should be believable but, more importantly, effective in lending themselves to elevating a particularly positive feeling.

As you do this exercise your RAS will evolve to notice related information to your affirmation sentence. This means you will start to automatically notice more of the small moments during the day which is associative and affirmative of your particular sentence. Reinforcing the habit and your gratitude in conjunction.