I had an idea but I can't remember it

The most impressive book on memory I have read is "Remember" by Lisa Genova.  She makes a point about forming memories: The reason I can't remember where on that large lot I parked my car is that I didn't form a memory of the location.  I can see that I need to get better at forming a memory. Having difficulty is the reason I try to carry pen (a miniature) and paper in my pocket.  I want to jot down a note of anything I think I want to remember.