For a certain amount of time (I suggest 50 minutes according to this study) do only the task at hand and nothing else (no phone, no email, no conversations, no Social Media and no leaving the building in case of fire). Nothing.
2. Wear headphones
You don’t have to listen to music, but wearing headphones will discourage possible colleagues, roommates and spouses to interrupt focused work.
3. Batch phone calls
Don’t always be available. Keep your phone on silent by default and return calls and answer messages in batches (e.g. 15 minutes in the morning and 15 minutes in the evening).