I struggle with these, but what works for me is thinking of this as a hill. The start is just where you put your boots on, the grind is the main part of the climb, at some stage on that climb, you realise that you have come further than halfway. I clear my desk, kid myself that I am organised and focused, then start and try to make it to 3/4 before allowing myself the option of doing something else I need to do (ie not my phone). I find that this makes it easier to get back to and I kinda find my motivation to get back and complete it, much more then if just 1/3 of the way through. When done, I also personally often find that my phone has less of an attraction.