Planning, organisation and self discipline! Remove the time "wasted" fighting fires, looking for lost things, etc. Don't let "to do" things pile up. Don't waste time constantly writing and re-writing "to do" lists when you could use the same time for actually doing something!
I am a fan of the "just do it" method advocated in one of the many personal career development courses I went on, which is basically a "one touch" approach to emails, snail mail, phone messages, etc. - only read/listen once and either deal with it there and then or delete/bin it. Not only does it mean you don't waste time reading/listening again at a later date, your head is clearer because having either done it or binned it, it doesn't clog up your headspace anymore, leaving you more headspace for other things.
More practical tips are things like putting all recurring tasks in a diary/planner/app, such as insurance renewals, utility contract renewals, road tax, MOT, etc so again you don't need to "think", as they'll automatically pop up when approaching due. Prepare shopping list templates on word or excel so that you don't need to have to re-write them every time you go shopping - just print out and delete the items you know you don't need. Same with packing lists for holidays - create a list with everything, print it out and then delete the items you won't need, i.e. swim wear and "sunny holiday" bits and pieces if you're going on a city break instead of a beach holiday etc.
Re children, get a good family planner and be disciplined to put everything on it, medical appointments, school events, school holiday and inset days, etc. Keep an eye on notifications from school whether paper, email or social media/apps etc and immediately note on the family planner anything relevant such as Mufti days, charity days, party invites, etc. That way you can "plan" in advance for things you need to buy/make which means less time than suddenly realising the day before and having to "waste" time to go shopping for just one or two items!
I know all that's personal/household/family related, but you can apply the same to your work and your studies. Plan your work/study working backwards from the deadlines and monitor your progress against the plan so that you can speed up if you're getting behind - much simpler to find a bit of time to get on top again if you're not far behind whereas if you find yourself seriously behind, it's harder (and less efficient) to catch up as you end up compromising/jeopardising other things to find the catch up time.