I’ve been wracking my brains on this and initially I started a list, but after half a couple of hours there's only one thing on it:
Don’t use the word ‘nice.’
That’s it. That’s the only thing I can recall. The teacher stood at the front, and said... and I can’t actually remember the exact words, though he did have a tendency to spit the word out with scorn. It was only when I read Messers Pratchett and Gaiman's 'Good Omens' that I came to understand the words real meaning, and how wonderfully useful it is to a writer
I like to use the word, partly in defiance of that now mostly-forgotten teacher, but mostly because in our world there are so many things that are nice. Not great, not earth-shattering, just nice. A hot cup of tea on a cold morning, that’s nice. An acorn-carrying squirrel dashing across the road, that’s nice. Taking out a summer coat to find a five pound note crumpled up amidst the faded receipts from last year, that’s nice. And as such, I vow to use the word in every book henceforth (or until I forget).