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.
Use the Zeigarnik effect to your advantage. If you want something to stop intruding your mind, write it down and promise yourself that you'll "deal with it later". If you want to keep pondering something (perhaps a problem you want to solve), don't write it down, and go for a walk with that problem on your mind.