ABC's to Staying on Top

In the past month or so, I've heard the same time management tip from 3 different people! I don't know where the idea originally comes from, but it is mostly common sense. I have found it really helpful in coping with anemia. But it applies to anybody, especially if you are very busy, or like me right now and don't have very much energy. It could even apply to how you use your money.
This tip works like this. Everything that I plan to do in day or a week fits into one of three categories.
A--things that MUST be done.
B--things that NEED to be done.
C--things I WANT to get done.
I need to do A before I do B. I need to do B before I do C. and I need to do both A and B before I do C. Within reason.
For example, today I have lots of washing. It is definitely in the A Category. Cooking dinner and staying on top of dishes is also A. Vacuuming is B. It needs to be done, but if it didn't happen, I could catch up tomorrow. I order my groceries every Monday, so that is also a need, but not a must as it could wait till tomorrow if it had to be. So I need to make those A's top priority today. Next in priority comes the B's. If there is any time and energy left, then I can do a C. If not, all the urgent and important things have still been done. Today I have been tempted to drop everything and go outside and mow the lawns. But that is not a priority today. I would use up all my energy on the lawns, and then be overwhelmed by washing etc. tonight. The lawns can be a priority another day. 
I am very good at dropping the A's and B's and doing the C's. Then it's the end of the day, and I'm worn out and overwhelmed by the things that still need to be done. I am impulsive and easily get distracted by C's or even B's when I should be doing the A's.
So this is a very good tip to keep me on track! It applies to all areas of life, not just housework.