Priorities can be divided up into:
URGENT - must be done now.
IMPORTANT - must be done soon.
UNIMPORTANT - must be done eventually.
There is, of course, a lot of overlap so you might also list the things you have to do only in order of importance - the top one or two becoming urgent.
Keep a diary and a pencil handy to record things as they crop up.
You could also use a card index, or even a computerised database, whatever you feel you can work with.
If you find that more than two items are urgent, then you are the victim of bad planning - do it better next time!!
MOST OF ALL
BE FLEXIBLE - LEARN TO FIT THINGS IN
BE EQUIPPED - DIARY, TIMETABLES,
Another way of setting priorities is to use the 4D system:
DUMP anything that does not need to be done at all...
DELAY what you can't dump...
DESIGNATE a time for what you can't delay...