If the service is excellent in a restaurant, call the manager over to compliment the server. If you admire someone's clothing, hairstyle, etc., say so! It makes everyone feel good and you never know when you will get another chance.
Years ago when the house I grew up in burned down, one of the lasting side effects of that trauma was a much deeper appreciation for the little things that make life so rich, along with less embarassment about expressing the positive. Tell people what you like TODAY. They may not be here tomorrow.