I like saving all my history, but I also find that it easily becomes a mess doing it in one file. So I split it up session, and then I merge it all into one file.

  # Eternal bash history.
  # ---------------------
  HISTCONTROL=
  HISTFILESIZE=
  HISTIGNORE=
  HISTSIZE=
  HISTTIMEFORMAT="%Y-%m-%dT%T%t"

  # read global history
  HISTFILE=~/.bash_eternal_history
  history -r
  # IIF there's no session history directory, create one
  if [ ! -d ~/.bash_history ]; then
    [ -f ~/.bash_history ] && mv ~/.bash_history ~/.bash_history.0
    mkdir ~/.bash_history
  fi
  # change history file to one for the session
  HISTFILE=~/.bash_history/$$
  # If it already exists, take a backup
  if [ -f "$HISTFILE" ]; then
    mv "$HISTFILE" "$HISTFILE"."$(stat "$HISTFILE" --format='%W')"
  fi
  # save the current session id + process id
  { export SESSION_ID="$(</proc/$$/sessionid)"; } 2> /dev/null
  printf "#$(date '+%s')"'\n# starting session:'$SESSION_ID' process:'$$' in:'"$PWD"'\n' > "$HISTFILE" &&
    history -r

  # Use trap to save history, not just on logout but also EXIT
  trap_exit() {
    # Append session history to global history
    ## Append to local history session to file
    cp -f "$HISTFILE" /tmp/history.$$.0
    history -a
    cp -f "$HISTFILE" /tmp/history.$$.1
    ## Append local history file to global history
    printf "#$(date '+%s')"'\n# ending session:'$SESSION_ID' process:'$$' in:'"$PWD"'\n' |
        cat "$HISTFILE" - >> ~/.bash_eternal_history
  }
  trap trap_exit EXIT