It's always on my mind.
It rings a bell.
I have a vague recollection of...
It's a the back of my mind
It has already slipped my mind.
It brings back memories of...
I remember it like it was yesterday
I recall it clearly
I intend to do it, but I'm not actively thinking about it.
I'm thinking about it all the time.
I've forgotten about it.
I remember the general experience but not the details.
It sounds familiar, but I don't remember the exact details.
When I see/hear/smell it, I remember...