Remember to say thank you to everyone, servicemen and women especially, but also anyone who helps you. You never know how you might change someones day or their outlook on life. I know from working with despondent people, people on the very edge of humanity that most people pretend don't exist, that you can change their day just with a smile and a nod.
That homeless guy on the corner with the sign? Smile and acknowledge him. You don't have to give him money, but sometimes a simple acknowledgement will go a long way towards him wanting to wake up the next day and try again.
The teenage cashier? You may have interrupted thoughts of how they were going to write their suicide note later after they got off of work. All with a simple smile and a thank you.
One of my favorite sayings goes like this "People may not remember exactly what you did or what you said, but they will always remember how you made them feel"