As others have mentioned, I use Post-It notes for book markers and other temporary uses. I've found other ways to record reminders. Having said that, I have used Post-It notes very successfully when trying to organize a major project for the first time.
My job happened to be a new position within the company and was being evaluated by outsiders coming in from around the state. Besides having my paperwork ready for the evaluation, I also needed to line up 3 community members to be interviewed, 1/2 dozen co-workers to be interviewed, have a breakfast ready, plus lunch.
Since this was my first experience doing this, ideas of every sort were popping into my mind randomly. Good ideas, but how to keep them
and sort them for usability?
My solution: 1. Retrieved a plain manila file folder and a pack of 1.5"x1.5" Post-It notes.
2. Every time an idea popped into my head, I jotted it down on the note and stuck it in the folder. This sure kept the brain "empty"

and relieved pressure headaches.
3. Great way to remember unrelated items like: send map to each visitor (days before internet was common), contact someone to act as hostess for breakfast and lunch at workplace.
4. When right side of opened folder was fully plastered, I glanced over all and gathered those related to one subject and put those together.
5. Spread subject groups across both insides of folder, writing subject above the notes.
6. Checked off each item as it was completed.
7. Checked off each subject title when everything below it was completed.
Evaluation Day came and I was as calm and cool as a cucumber. Everything was perfectly planned and went off without a hitch.
I have since used that same method when planning other areas of my life that have many parts to organize, no matter how insignificant they may be. This could be used to plan a party, a wedding, a trip, a holiday, etc.
Nowadays people would organize on the computer, but I am one who still needs paper and pencil for certain tasks.