"How do I set priorities?" Because I hear that so frequently, I assume most people think they could and should be doing it better.
I have a radical point of view: learn to listen to, and trust, your heart. Or your intuition, or your gut, or the seat of your pants, or whatever part of your anatomy is the source of that mysteriously wonderful "still, small voice" that somehow knows you better than you do, and knows what's better for you, better than you do.
The ABC/123 priority codes don't work. Listing your top 10 things you think have to get done, in order, doesn't work. You'll have a different priority set at 8PM tonight than you will at 10AM this morning. And sometimes the most strategic thing for you to do will be to water your plants. Like, when you've been in six meetings, felt beat up in five of them, and by 4:30 your brain is scrambled eggs, and you barely have the attention span of a gnat. That's the time to water your plants and fill your stapler. Why? Because you can't do anything else, and you're going to have to water your plants sometime anyway.
On a day to day, moment to moment basis, there is no algorithm or formula that will last very long, or is really worth trying to nail down in some written or coded system. The four criteria that you will use to decide what to do are (in order of precedence):
Context (what can I do where I am?)
Time (when do I have to do something else?)
Energy (how fresh/wasted am I?)
Priority (what has the highest payoff for me if I do it?)
The idea of "payoff" to yourself is the intuitive one. But let's not be frivolous--when was the last time you and your professional colleagues (or your family) as a group took a sincere look into the future and made the hard decisions about what is still mission-critical and what is not? When did you last decide what your job really is? When was the last time you personally sat down and thought through where you are in your life, on all fronts, and where you're going, and what you really want to be different than the way it is?
The best to shoot for is a regular enough revisit to the broader horizon perspective for your work and your life, letting it sink in at all the levels (conscious and otherwise) it might affect. Then get organized and current enough with your current realities and commitments, so that you have a clear enough deck to listen to the internal directions and hunches, and to follow them without distractions.
Do I work on this article I need to write? or call Aunt Susie? or balance my checkbook? or plan the next year's marketing strategy? or have a beer and hang out with my spouse in the yard? Who knows?
Be open to your own intuition and its directions -- then take the risk to move on your best guess, pay attention to the results, and course-correct as you keep moving along.