Show, don't tell.

You have heard this several times before, but it is one of the most important writing techniques you should employ and bears repeating. Using dialogue and action to convey emotion is more powerful and interesting than narrating it. In other words, don't write that Jack furrowed his brow; convey his concern with the language he uses when speaking to another character.

User correct punctuation.

Professional editors, agents, publishers, and readers will get distracted by your unnecessary insertion of en- or em-dashes, overuse of commas, and misplacement of semi-colons. Unless you're writing something meant to be highly experimental, don't invent your own punctuation rules. The ones we already have work just fine.