The secret to mastering your time is to systematically focus on importance and suppress urgency.
Humans are pre-wired to focus on things which demand an immediate response, like alerts on their phones—and to postpone things which are most important, like going to the gym.
You need to reverse that, which goes against your brain and most of human society.
Look at what you spend your day doing. Most of it, I'll warrant, is not anything you chose—it's what is being asked of you. Here's how we fix that:
Most of us follow an implicit social contract: when someone asks you to do something, you almost always say yes. It may feel very noble, but you just agreed to slow yourself down because you were asked nicely. You may need to sacrifice some social comfort (as a bonus, people tend to instinctively respect those who can say no).
Unplug the TV.
I haven't had a TV signal for 7 years, which has given me about 12,376 hours more than the average American who indulges in 34 hours a week. I do watch some shows—usually one hour a day whilst eating dinner—but only ones I've chosen and bought. You can do a lot with 12,000 hours, and still keep up with Mad Men.
Turn notifications off.
Modern technology has evolved to exploit our urgency addiction: email, Facebook, Twitter, Quora and more will fight to distract you constantly. Fortunately, this is easily fixed: turn off all your notifications. Choose to check these things when you have time to be distracted—say, during a lunch break—and work through them together, saving time.
Schedule your priorities.
Humans are such funny critters. If you have a friend to meet, you'll arrange to see them at a set time. But if you have something that matters to you more than anything—say writing a book, or going to the gym—you won't schedule it. You'll just ‘get round to it'. Treat your highest priorities like flights you have to catch: give them a set time in advance and say no to anything that would stop you making your flight.
First things first.
What is the single most important (not urgent) thing you could possibly be doing? Do some of that today. Remember there's a limitless number of distractions—don't fool yourself by thinking "if I just do this thing first then I can."
Less volume, more time.
There's always millions of things you could be doing. The trick is to pick no more than 1-3 a day, and relentlessly pursue those. Your brain won't like this limit. Other people won't like this limit. Do it anyway. Focusing your all on one task at a time is infinitely more efficient than multi-tasking and gives you time to excel at your work.
(You can outsource your to-dos to Fancy Hands, of course!)
It's rude, unprofessional, and often utterly necessary. There are people you won't find time to reply to. There are requests you will allow yourself to forget. You can be slow to do things like tidy up, pay bills, or open mail. The world won't fall apart. The payoff is you get done what matters.