I just finished reading Following the Whispers by Karen Walker. According to her memoir, she wrote in her journal constantly. I'm sure this helped her remember details when she began to write her book. (I'll post my review on it in a few days.)
Many of us aren't ready to write the official memoir but we can still keep a journal. I used mine extensively when I wrote Uncut Diamonds, my autobiographical novel. (I don't think I could ever write a true memoir. I like to make up stuff and change situations to suit the story.)
If you're not used to writing in a journal, it can be awkward at first. Especially if you think about "your posterity" reading it. That idea can really tie up the flow of words. I prefer to think of journaling as my thing, something I do just for me. I get a nice, smooth-writing pen, sit in my chair, shut the door, grab a cold diet Coke and just write stream-of-consciousness about what I did, what I'm going to do, what I'm thinking about. Not to mention dumb stuff like how much I weigh, who I'm mad at, what frustrates me and how much I spent at the store.
I don't write for my grandchildren and great-grandchildren who may or may not read them someday far in the future. If anyone ever wanted to go through my 4000 journals, which I doubt they would, then that's their problem. They'll have to decipher the handwriting and figure out who I'm talking about when I say "S. was a b. today" or "I hate mean people."
Really, journal writing is the best and cheapest psychotherapy ever. If for no other reason than to vent, or plan, or talk about nothing to someone who cares (yourself lol!) I highly recommend it. And when you want to write that memoir, look at how easy it will be with events of your life already recorded.