Use a unique password and change it every 3-6 months. If your billing supports 2FA, enable it through the authentication app.
Keep track of devices in your personal account. Remove a set-top box that you haven't used for a long time to free up a slot and avoid accidentally sharing access.
If your device is lost or stolen, immediately write to support and ask to reset the session. Also request a data download (GDPR) - this will allow you to see which logins have been performed recently.