Keep a notebook (not a DOC file in your computer) where you write down everything that you've done in detail and that you need to do, in what order of importance, which you can reference easily if you screw up and need to fix the problem. It helps you trace your thinking and workprocesses, keeps you neat and organized. Keep a clean desk. Put things in places where you know you can always find them and within easy reach. Your desk reflects your state of mind. If cluttered, you're not focused and can't do a good job, almost by default, no matter how smart or how good a memory you have. It's just a matter of time. Clean your desk regularly - once a week, at least, ideally.
Make your desk a comfortable and ergonomic place. Sit in a comfortable and breathable chair. Make it your own with pictures of family, photos of favorite places, and other positive associations. Remember, you spend a third of your entire working week here, if not a lot more. Even if you hate your job, at least you'll have constant reminders of why you're doing this (or of why you should get out of there immediately). If you're allergic to dust, for G-d's sake, don't be lazy and clean the dust off yourself ASAP! Don't wait for the janitor to do a thorough job.
It's important to remind yourself every day why you are working where you are - hopefully because you love your job, but likely also to pay off loans and debt, to save up to buy a house or to travel or for some other worthy goal, and even more so, to provide for your family. This thought process helps to bring you through the tough times, the dark days, the days when you want to quit and not look back. Be pragmatic. Don't let your cubicle or office stay drab or dirty. Control as much as you can in your work environment, so you can deal well with whatever you can't control.
Set your chair in front of your monitor(s) very carefully and symmetrically so as to minimize straining the neck.
Adjust the seat level so that you are above the keyboard, ideally, to minimize wrist strain or worse, the chances of carpal tunnel. Buy a wrist pad or order through work.
Adjust the screen(s) to minimize eye strain. Don't over- or under-do it on contrast level or brightness.
If you're on the phone a lot, order a headset.
Take regular breaks - every 45 min-hour to get up and walk around and/or stretch, ideally outside and in a green space. Stand stably with your two legs slightly out. Fold your arms in slightly at the elbows and swing to one side, then the other, back and forth, several times. Straighten your back and stand properly upright. Breathe in and out deeply several times. Stand up on your tiptoes and stretch your quads, up and down; several times. Repeat as many times as necessary/desired to stretch out your skeleton and muscles.
Take walks to break up the morning and the afternoon. It not only helps to refocus you and get your mind off work for a few crucial minutes. It also helps to wars off boredom, creates novelty and improves your health, if practices regularly.
***REMEMBER, SITTING IS SLOW DEATH*** Numerous studies have shown that sitting for most of the day, as the great majority of office workers do, leads to higher rates of obesity and cardiovascular disease, diabetes II and other chronic diseases. YOU MUST DO EVERYTHING YOU CAN TO FIGHT OFF THIS SLOW DEATH, MOST IMPORTANTLY TO MOVE YOUR AS REGULARLY THROUGHOUT THE DAY!
As you sit, straighten out your back and breathe in deeply, sucking in your abs. Hold your breath for 10-15 seconds at a time. You're now stretching your muscles and re-arranging your internal organs slightly, which is important.
Maintain your vision by taking frequent breaks to blink and re-moisturizer your eyes. Choose a point far in front of your monitor to focus on and do so on your eye break.
***Productivity at work is NOT a function of hours spent in a row looking at a spreadsheet or focusing on a problem. It is a function of organization, good planning, taking good care of your body throughout the workday, taking frequent breaks, maintaining a routine full of movement and change in tasks.
***80/20 PRINCIPLE. 80 percent of your most important work is done in 20% of your time. Plan accordingly to attack this work when you're at your freshest and most energetic - in the morning, for most people. You'll feel super accomplished and will get much more done and on time or ahead of time, this way.
Know your body and how it functions and when it functions best and when it functions worst. Apply the principles of self-observation, monitoring, A-B testing, deduction, induction, iteration and adjustment, in series.