You can always give the forgetful a perpetual birthday calendar to help jog their memories... I use one (when I remember). Once it is filled in, you reuse it each year because it is not year-specific and just lists the birthdays/anniversaries each month.
Like this: http://www.papergraces.com/gbd-a006.html
Like this: http://www.papergraces.com/gbd-a006.html