One research study asked students to think about an important exam. Half of the students were asked to put in writing specific plans of what/where/when they would study. Later, all students were asked to do a word association test. The group of students that did not write any study plans produced more word associations related to studying because studying was still on their mind; the group who did write down their study plans did not exhibit a comparable bias during the word association test.
If you are planning something important, preparing a presentation, or even writing fiction you can use the Zeigarnik Effect by stopping writing in the middle of an idea or passage. That way it will play on the mind, in a pleasant and creative way, and it will be much easier to resume writing next time.
Conversely, we can use the Zeigarnik effect to our advantage by deliberately keeping unanswered questions in our mind. We can ruminate about them, even when we do something that has nothing to do with work and ideally does not require our full attention