Conducting interviews really helps put things in perspective. Or, once someone is hired, attempting to delegate some of your work to them.
Alternatively, try picturing what you would do to train people for the work you do. Think about people you know (even marginally) that are in the same field as you, and then try to figure out who could do as good of a job as you, or identify what they would have to learn. It's in scenarios like this, you realize just how much you know - not just about your particular job, but about your field in general.
On the flip side, you really don't want to be irreplaceable, and if you find yourself doing a lot of work because you think it will take longer to explain the task than to do it yourself, then you've got other problems to work on.
Alternatively, try picturing what you would do to train people for the work you do. Think about people you know (even marginally) that are in the same field as you, and then try to figure out who could do as good of a job as you, or identify what they would have to learn. It's in scenarios like this, you realize just how much you know - not just about your particular job, but about your field in general.
On the flip side, you really don't want to be irreplaceable, and if you find yourself doing a lot of work because you think it will take longer to explain the task than to do it yourself, then you've got other problems to work on.