Not doing a million other things at the same time, like checking email, watching a bit of the news, checking social media, texting with friends.
To start off, your writing time is for writing. Not doing a million other things at the same time, like checking email, watching a bit of the news, checking social media, texting with friends. My recommendation, before each writing session, is to clear away all other distractions. Shut down all other windows on your computer, put your phone on Do Not Disturb, or better yet in the other room. Shut down all notifications from any apps on your phone or computer. Turn off the TV. Clear your desk of clutter. Let your writing time be about WRITING.