Every summer during college, I diligently worked at Papyrus, a frou-frou paper goods store at the local mall. So I was mortified when I took a spelling test for my first job after college (generally a requirement when you’re applying for a copy editing position) and missed the question about the difference between stationary and stationery. I memorized this hint and have relied on it ever since.
If you’re referring to paper goods that you write letters on, the correct spelling is stationery. Remember that by thinking “e for envelope.” If you mean standing still, use stationary.