Every so often, I will remember the fact that every day could be my last and that I need to seize the moment and get busy living.
The trouble with these moments is that there seem to be a lot of things on my "try to do before you die" list. The result tends to be a rather overwhelming set of tasks that I suddenly remember I need to get to work on (write another chapter, no a blog post, no say a Rosary, no try to meet up with a friend, no write another e-mail to *redacted*, no practice Italian, no...).
I tend to over think things, especially 'hypothetical' or theoretical (or whatever '-etical' you would call it) propositions. If someone says "imagine this is your last day on Earth..." it doesn't really motivate me because I think of what I would do if I literally knew this was my last day on Earth, and those things definitely wouldn't include going to work or writing another chapter of a book I'd never finish.
More useful is the simple awareness that there is no gaurantee of tomorrow...or, heck, even another hour. Though, again, this tends to put me into panic mode where I try to think of all the things I want to do and it's like they get stuck trying to get through the door of my mind ("The three-stooges syndrome").
Momento Mori is like the nuclear option of motivation: the tricky part is channelling the energy productively, rather than letting it just spread out in confusion and eventually dissipate.