My personal productivity is often dependent upon other people completing tasks as promised. For example, my doctor promises to send an important health certificate to a third party so that I can be granted something.
But I too often find that even professionals simply don't do what they've promised, and I usually find out about their failures at a late stage. This causes huge problems for me --- my productivity is disrupted or even breaks down. At the same time, I don't want to constantly call people verifying that they're doing their jobs.
What is the best way of handling external factors like this in your private lives, to minimize the risk that somebody fails to deliver as promised?