The distinction between stationery (pens, pencils, binder clips and other office gear) and stationary (not moving) is the first spelling mnemonic I can recall learning. However you remember it, it's important to get it right.
Stationery picture from Shutterstock
I was reminded of this common error by Lifehacker's own Chris Jager, who messed up and used the wrong version in a post last week. As a kid, I was taught that you wrote a lettER with stationERy. The only problem there is that letter writing is on the decline.
Another possible approach is that papER is a kind of stationERy (we'd welcome additional tricks in the comments). Choose the one that works for you. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.