Believe it or not, YOU are the one that decides whether or not to be happy.
1-Live a healthy lifestyle.
So a great way to keep a smile on your face is to exercise, eat right, drink lots of water and get plenty of sleep. I think this is a no brainer, but very important none the less.
2-Stay focused on the positive.
Another one that is very obvious and yet not so commonly implemented is to stay focused on the positive. No matter what the situation there is always a silver lining. Find the good that is always present and keep it at the forefront of your mind.