Long ago, I heard this advice on some ancient management tape: "never let a piece of paper cross your desk twice". It stuck with me and I've got into the habit of immediately answering, archiving or on occasion setting a snooze timer (in GMail). If the recipient takes my promptness for granted, then I might adjust my policies for just that person. (Un)fortunately, it helps to be a manager -- then you can give feedback to people who don't read emails properly.