I have adopted one convention: If I need to act on something later, I put a box to its left. Once I acted on it, I tick it off with a checkmark.

For example, during I a meeting I realize a problem in our software which is unrelated to the actual meeting agenda. I jot down a box and a few words in my college notepad. Later, when I have a few minutes, the "few words" are expanded into a proper ticket in our issue tracker and the box is ticked off.

I strive to tick off every box in my notepad by the end of the day. Sometimes that just means writing it down elsewhere (Email, electronic todo list, OneNote, Jira, etc).