According to Alan Webber (a Harvard Business guy), a set of 3 x 5 cards is one of the most important and successful tools anyone can  use.

Coordinate your set with one of our fully leather-lined 3 x 5 card binders. Choose your ring size (¾ in. and 1 in.), your color(s), your extras (pen loop? pocket?), your closure, and begin managing your  great ideas (or recipe cards, wine tasting notes, fishing logs, etc.)  in style. And with three black dividers included, you can organize work, play, and more play.