Be thankful. Express your gratitude. Keep a gratitude journal. Gratitude has become to mental habits, what eating veggies is to dietary habits. You've heard a lot about it, you know how important it is, and either you already do it, or you feel bad that you don't.
The commonly recommended practice is to take a few minutes before bed and jot down a few items from the day that you're grateful for. This practice is thought to make you happier, healthier and less stressed. Oprah recommends it, as do many other experts and gurus.