Contrary to what we'd like to believe, multi-tasking is actually a time-waster. At work, if you spent half the day multi-tasking among 2-3 different tasks, 45% of the time actually goes to waste. The mental adjustments - like re-orientation or re-familiarization - that has to happen every time you switch from one task to the other actually reduces your over-all productivity. Imagine, if you did that all day on your 8-hour work day, that's 3.6 hours wasted productivity. If you're a business owner with a workforce of multi-taskers, it's a nightmare to realize that you are getting 45% less productive work out of your people, and that's a terrible waste of money.
Here's the hack: Resist the urge to multi-task! The temptation in multi-tasking lies in the fact that by doing different tasks simultaneously you feel you are accomplishing more (quantity), when in fact, this compromises your real productivity by not only wasting time but also diminishing the quality of your work.
Try finishing one task first, take a quick break, then move on to the next task. Doing this not only allows you to accomplish more, it also ensures you do a much better work.
Admittedly though, sometimes the multi-tasking happens because the distraction of other tasks crop up without your control - like an email that sends a ding and gets your attention, or a text message or phone call that demands to be answered, or a co-worker that passes by for a chit-chat. Avoid these distractions or anything that steals your attention - put your phones on silent mode, or put up a Do Not Disturb sign on your desk, or work in an area where you cannot easily be distracted, or always wear your headphones (it dissuades people from disturbing you because it makes you look occupied).