I don't necessarily do a good job in the above. However, surprisingly perhaps, I have never felt that my professional commitments have played a major part in ever tiring me out. But when I club that with many interests and hobbies, family commitments, social interactions etc., sometimes it gets overwhelming and I just lose it.
But when I am managing well, I think the following are the mechanisms:
Concentrating on one. Even if there are many things to do, it helps to forget about all others for a while but the one immediately at hand.
Learning to let go. It's important to realise that since there are too many good things to do in life, we are anyway going to end up not doing most of them. So, it's best to pick a handful and stick to them.
No hurrying. I feel seriously fatigued if I have to do anything in a hurry. Doing things in a soft pace allows me to get into the flow and enjoy the job.
Time-management. All standard time-management techniques: planning, scheduling, tracking, discipline. Also, where possible, I don't try remembering anything. My life runs on zillions of reminders on phone, Lotus Notes, gmail, yahoo...my wife ;)
Breaks: Getting up once in a few minutes, and taking a day off every now and then. I don't think I tire out more these days. But I have recently learnt to acknowledge fatigue. Also, I have stopped denying my body and mind a well-deserved rest. Well, most often.
Fitness. I am not a fitness guy. But as years go by, I seem to be tending a bit towards it. I would like to think that I have several years of active life ahead of me. A sick body is a big impediment. A fit one is an instrument to carve out higher objectives. So, a bit of exercising: walking, jogging, cycling, swimming, yoga...whichever I feel inspired to do. I have moderate dietary habits. So, that's taken care of, more or less.
Masochism. Over the years, I have developed a taste for the torturous feeling that one gets on seeing hordes of incomplete assignments. A half-read book, a half-written story, an incompletely developed software, pending research, incomplete drawings...earlier used to drive me mad. These days, I feel at home with this predicament. I don't feel uncomfortable even with the idea that when I die, I would be having plenty of unfinished stuff to do with me. The ability to add a tiny bit every once in a while -- taking a tiny steps towards the destination of a long journey -- often gives me a heady feeling.