I often notice the words stationery and stationary used interchangeably, and wish the writer had been fortunate enough, as I was, to have a teacher explain a useful mnemonic for remembering which is which.
(In researching this post, I've come across a fun site called Mnemonic Dictionary where people suggest their own mnemonics for various words.)
The trick I was taught for distinguishing between stationery and stationary was similar to the one at HumbleApostrophe.com.
But I think the best way to distinguish between spellings is to know the reason behind them. A stationer, in mediaeval times, was a merchant who had the right to stay in one place rather than have to wander around, as pedlars usually did at that time. The stationer was usually based near a university and sold books - but also writing materials.
So, if I just remember that there's a person selling paper and writing products in the shop, and that person is a stationer, I should remember the spelling of stationery.
The Worshipful Company of Stationers and Newspaper Makers in Britain has a little more about the topic. (I love that name.)
And more at Ryman Stationery in the UK.