I create an Excel Spreadsheet so I can record a brief summary of each chapter as I go along. It is an invaluable memory-jogging tool which I frequently draw on. Trying to do this in hindsight is far more time-consuming. I also make a note of the number of words in each chapter to give me an idea of the running total. I’ve been meaning to buy Scrivener, which some members of my writing group swear by, but this is still on my ‘to do’ list . . .
Sometimes I keep a timeline so I know exactly when each event happens. This helps continuity.