This is already discussed in TimeManagement - merge the pages?
A ceaseless array of requirements and priorities flows into your life from the future. How do you decide what to do today?
others?
What is the most important thing?
Do that.
Ignore everything else. Pretend that nothing else exists.
Um, yeah, great. Now if you can tell us how you go about deciding what the most important thing is ...
... deciding what the most important thing is ...
Well, given that this will always be subjective, figuring it out for a daily routine is not so hard if you are able to determine what the long term important thing(s) is(are).
If the most important thing is to "GoToTheMoon?" (arguably a long-term project), then today's "MostImportantThing?" will be whatever contributes the most value toward accomplishing GoToTheMoon?.
Deciding what your GoToTheMoon? goals are ... well, that's a matter of personal inspiration, desperation, or whimsy.
LifeVectors has a model for how it breaks down, but establishing the goal itself is up to you.