"Q" reminds you that Questions always need question marks.
"U" and "..." together to suggest Unusual and Interesting vocabulary.
"C" reminds you to check Capital letters at the start of sentences and proper nouns.
"K" is for different Kinds of sentence (a bit of a cheat, but varying the sentence length and subject avoids predictability and helps writers achieve a higher mark).
"S" tells you that Speech always needs speech marks (I'm hoping they'll check spelling anyway).
"P" is for the all-important Paragraphs which can even be marked in at the end, if need be.
"A" reminds you that Apostrophes are needed for possession and abbreviating.
"M" gets them checking for a Main verb in every sentence and not just writing in note form.
I encourage pupils to spend the last five minutes of writing time doing a QUICK SPAM check - right up to GCSE, it gets them actively proof-reading for errors and can help identify what they need to do to improve.
John Gallagher Teacher of English, King Henry VIII School, Coventry