This works for me, if I’m in the shower, at the gym, driving or somehow can’t write down things I want to do. Then I just remember the number of things I want to remember (until I get a chance to write them down). Remembering this number increases the odds of remembering everything I was thinking of (things to do/say, ideas, etc.).
For example, knowing that you wanted to remember 5 things will help you if you remember 3 of them easily - because you have connected these 5 things in your brain thinking about the 3 items/tasks might help you recall the rest.