1. Make sure that you have your goals and objectives in front of you. Use a whiteboard or other visual aid to keep your big picture aims where you can see them.
2. Check that your daily tasks are mostly aligned with your goals and objectives. A lot of "small stuff" drops into our day that either isn't aligned to our aims, or we rationalize into "alignment".
3. Even though we all like to be seen as helpful, be careful about taking on too many tasks, or tasks that aren't aligned to mission, vision, values, goals, and objectives.

All the newest research says that multi-tasking does not work. So, you need a system to get the most work done. I recommend you take a look at David Allen's book "Getting things done". Its a very workable system. I have also heard good things about another book called the Pomodoro technique.
good luck!

The hardest thing to manage is priorities, so if your environment thrives on the crisis de jour, and whiplash planning, and reprioritizes projects tasks and customers, your problem is defining and implmenting a priority management system that all affected parties can agree to. Organizational change is hard to manage but chaotic priority shifting lowers effectiveness and productivity more than most contributors to the chaos.

People all have their own innate capacity for work, gained through learning what works for them to assist them to maintain a level of contributions output for sustained periods. But it is encumbent upon the company to staff and manage workloads according to the the capacity of your people. In some circumstances people can,for short periods, increase their capacity to do slightly more, but it will degrade their own capacity and require a resting period almost like any athlete in training to return to their normal capacity. Sprinting in work also may affect the quality of outputs and is a consideration.

STRESS ADAPTATION SYNDROME

János Hugo Bruno " Hans " Selye CC ( / ˈ s ɛ l j eɪ / ; Hungarian : Selye János ; January 26, 1907 – October 16, 1982), was a pioneering Hungarian-Canadian endocrinologist of Hungarian origin. He conducted many important scientific work on the hypothetical non-specific response of an organism to stressors . Although he did not recognize all of the many aspects of glucocorticoids , Selye was aware of their role in the stress response.