Learning the difference between 'principle' and 'principal' requires only a simple and cheesy aide-mémoire: "The principal of your school is your pal." Clearly most people remember their school principal as an evil cane-wielding figure, because this error still occurs regularly. Check out the Back To School sale advertising from Myer above.
Myer apparently changed this signage after customers pointed out that it was incorrect. That's a good thing, but it would have been better to avoid the mistake in the first place.
Admittedly, the 'principal/pal' trick isn't entirely helpful, because it doesn't cover the frequent use of 'principal' as an adjective meaning, roughly, "first or most important". It's yet another case where, ultimately, you need to learn the correct usage in each context.
Accuracy matters. That's a principle no-one can argue with. Thanks Kaaren for the photo tip!
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.