Think Negative

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. 

...They (the stoics) recommended that we spend time imagining that we have lost the things we value—that our wife has left us, our car was stolen, or we lost our job. Doing this, the Stoics thought, will make us value our wife, our car, and our job more than we otherwise would. This technique—let us refer to it as negative visualization—was employed by the Stoics at least as far back as Chrysippus. It is, I think, the single most valuable technique in the Stoics’ psychological tool kit.
— William B. Irvine, A Guide to the Good Life
...for example, when we kiss our child, to remember that she is mortal and not something we own – that she has been given to us “for the present, not inseparably for ever.” His advice: In the very act of kissing the child, we should silently reflect on the possibility that she will die tomorrow.
— Epictetus

Live in the moment and enjoy the now. We get so caught up in our own busy lives that life literally passes us by.

Life passes most people by while they’re making grand plans for it.
— George Jung

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:

Man. Because he sacrifices his health in order to make money. Then he sacrifices his money to recuperate his health. And then, he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived.
— Dalai Lama

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?