1. Work out of a separate office, ideally with a door. Don’t go in that room except when you’re working. If you work from a laptop, (mostly) never take it out of that room. Obviously some don’t have this luxury, but if you do and you’re not doing this, I highly suggest trying it.
2. Remove work communications from your phone. If you have a separate email account for work (highly likely), remove it from your phone. If you have Slack or some other chat app installed for work, remove it or remove your work account. If your company truly cares about a healthy work-life balance, this shouldn’t be an issue. If you need it for oncall, only install it then, then remove it.
3. Stick to a schedule. Log on and off about the same time each day. Don't jump on work for an hour or two here and there throughout the whole day. Log on, put your time in, log off. Don't work on weekends. There are times when you may need to put in extra and/or odd hours, but this should be the exception, not the norm.
With these three things in place, I am — quite successfully — able to get the pre-smartphone feel of disconnecting from work while working from home.