There are certain lessons—and when I say lessons, of course, I mean grammar lessons—that we always remember having been taught, and this is one of them for me.
I still recall my elementary-school teacher explaining the difference between stationary, which means to be still, and stationery. She said that a good way to remember that stationary (with the a) means to be still is to think of the word stand (with the a). That’s good since when you’re just standing, you’re not moving (that is, you’re stationary), but I would go one step further: Stationery (with the a in the first part of the word and the e in the second part of the word) is paper (with the a in the first part of the word and the e in the second).
I wonder if anyone will remember that one 40 years from now!