Do not sit down at your computer without a plan.
If you don't know exactly what you're going to work on, the time will inevitably disappear into social media and internet neverland.
What will you spend your time doing? Do you have a list of tasks you'll tackle? What needs to be done?
Not sure how to come up with a plan? This is how I organize my blogging time. (That is, when I take my own advice.)
That's it. Simple.