They said it was either Up+B or Down+B, but I don't quite believe it, as I've never gotten it to actually work. It's easy to say "Oh, this trick is really hard, and if you don't manage to do it, it's because you don't know how to time it", because no one can prove that false, but my opinion is don't waste your time. Go for the Missingno. trick instead to duplicate real Master Balls if you have the Red or Blue Versions. Pity Missingno. doesn't appear in Yellow, though.