I always immediately write down what happened in therapy and what I'm feeling in the moment as soon as I leave session -- otherwise I will absolutely forget. If I were to only rely on my memory I would remember absolutely nothing. But by writing it down and occasionally going back to read over things, I remember somewhere around 75% of what happened.
Although there are always things I forget the second I walk out of the door! And it's hard to even write down things afterward if I was dissociating in session because then I remember nothing.