I use IMDb ! Everytime I watch a movie I give it a grade and then if I go to "my vote history" I can see all the movies I've watched ! And I have an Excel sheet of my favorite 250 movies and an other one with the countries/years/actors/directors most represented...
IMDB, as theoffice911. A dictionnary of cinema when I can notice every classical movie I have seen. And I used to fill a notebook, that I lost few years ago... And since few months, indirectly, Whatthemovie ! It even helps me to remind of bad movies I have seen many years ago !
Like forerunner I use my memory ... I've started to rate movies on imdb only recently (but thanks for the tip, theoffice911!).
An excel sheet wouldn't help me with a screenshot I recognise vaguely and can't identify but where I know I should know it - sooner or later, however, it will slip from the tip of my tongue, IF I'm lucky ;-)
From memory, but I watch and re-watch movies all the time; each time I see something new. Even movies like the LotR trilogy, or even Star Wars, both of which I've seen a gazillion times, I still see new stuff. :)
I also started keeping track of movies I've seen at IMDB (only since I play whatthemovie)
I don't use vote history though, I add titles to "My Movies", I created folders for movies I've seen and for movies I still want to see and put the into the folders. I usually only add movies where it's likely I'll forget the title sooner or later. I don't add stuff like Shawshank Redemption or Star Wars for example :P