Avoid contractions, colloquial forms, repetition of forms (Idid this, then I did that. Next I did the other). Don't start every single sentence with a connector. Avoid one-sentence paragraphs- either an idea should be developed, or it should added elsewhere. I hate direct questions- you're supposed to be answering them, and I find it annoying when people start on the 'have you ever thought...' stuff. Exclamations marks are the kiss of death in a composition for me- someone once said that they were like laughing at your own jokes- your reader shouldn't need a guide to emphasis if it's well-written.
- For Teachers