Say "thank you" to people.
Say it when someone offers you a seat on the bus because they've noticed how tired you look.
Say it to the security guard who holds open the door for you when you enter a store.
Say it to the cab driver when you reach your destination.
Say it when your food arrives in the restaurant, even if your waiter dumps it unceremoniously on the table.
Say it when someone helps carry your heavy bags up a flight of stairs.
Say it when the postman hands you your letters.
Say it when your greengrocer helps you pick out the ripest tomatoes from the pile.
Say it often and say it loud enough for the other person to hear. Don't mumble and expect them to magically understand what you meant. Acknowledge the deed and express your appreciation.
Say it even if you know the other person will snort and look at you derisively. Say it unabashedly.
Say it when someone goes something kind for you, whether it be a choice or an obligation on their part.
Say thanks often. Say it with a smile on your face, even if you don't feel like smiling. Make it such a deeply-ingrained habit that after a while not saying thanks makes you feel uncomfortable inside.
Showing your gratitude never hurts.