Any tips for staying consistently grateful? via /r/gratitude

I feel grateful in moments of focusing on what I am grateful for, and everything is rainbows and unicorns. But then I move on with my day, and the doom and gloom returns. I forget everything that I was previously grateful for and get frustrated with myself and life in general. How do people stay consistently grateful, forever able to go back to a place of gratitude even when life gets you down? I'd love any help and advice, please.