As a general principle, spend 80% of your time on the 20% most important activities.
Don't spend more time maintaining your to-do list than doing what's on it.
The corollary to this is that 80% of what will fall onto your plate is just not a priority. That doesn't necessarily mean it can all be discarded, but some of it can and rest can be postponed until the important stuff is done.
Put an index card up on the wall in your office that says
80 / 20
to remind you.