Counter-intuitive, right? Negative visualization is one of the most useful techniques born out of ancient stoic philosophy. Simply put, the best and easiest way to appreciate the life you have now, is to imagine your life without the things and people you cherish the most. Your beautiful kids - gone; that nice house - gone; etc. Try it. It hurts, but in a gratifying way - like being sore from a great workout.
Live in the moment and enjoy the now. We get so caught up in our own busy lives that life literally passes us by.
A great observation was made by the Dalai Lama when he was asked what surprised him most about humanity, and his response was mind opening:
So, cherish the things you love by imagining your life without them and enjoy the life you have now by contemplating your own mortality. Don't take your life for granted and don't let it slip through your fingers while you sit and hope for a better tomorrow. Enjoy it now. Do you really want to die having never really lived?