The way I work is to get things off my plate as soon as possible and move on to the next task. The problem is it is now filed in the back burner of my mind. It is still hovering in the background somewhere, but I've completed the work, done whatever I needed to do with it and diarized whatever needed to be diarized and moved on until I need to know it again. My boss may come out a month or so later and ask me questions about it and sometimes my mind initially draws a blank. I need a minute to go back and check and then I am completely up to speed again.
I find with the volume of work admins deal with throughout the day, week and month, we need some system to keep things straight. For me, the thing that helps is to document everything . I put items in my calendar as a reminder, but the problem with calendar reminders is unless you know which day you put it on or what you named it, you can't easily find it.
For each meeting or event, I create what I call a meeting sheet. Most everything I do as an admin revolves around meetings. I'm either organizing a meeting or responding to someone else about my boss's availability. When I receive an email relating to a meeting, I create a meeting sheet and cut and paste the email in there and make any relevant notes. If I put a reminder in the calendar, I note the date on the meeting sheet. If I am waiting to hear back from someone, that is documented as well. It started out that the meeting sheet was just for my information, but now I use it as a filing of sorts about each meeting.
This system keeps me straight. I know others who keep folders in their Outlook account and drag and drop emails that have to do with a particular meeting. Whichever way you do it, you need a way to keep track because ultimately your boss will come back and ask where you are at with a meeting or want to know the background of how they got where they are.
I do the same for all my tasks. I document what I need to do, I document what I've done so far and I document what I need to do to get me to the end. This way I'll never forget and can come back at any time to see my progress.