People who smile often tend to live happier lives. When you're feeling down or like nothing is going your way, try smiling instead of staying stuck focusing on what's not going well. The act of smiling literally makes your brain release endorphins that help shift your mood in a positive direction; the key is to smile when you normally wouldn't or don't feel like it.
The next time you're going through a difficult time, remind yourself to smile three times (more if you wish) and see how it makes you feel. Hold each smile for thirty seconds so your body has time to produce the endorphins. You'll notice your mood change and a general sense of well-being.
I've actually met people who say they don't like to smile or were taught not to smile; guess what kind of lives they had? An important thing to understand about smiling is that it isn't some horrible or frivolous activity, it's just part of learning to be happy with yourself and enjoying life.