This checklist has all of the main points, and is also appealing to look at. It covers everything from the correct form of your/you're, their/there/they're, and our/are, to correct tense, adding sensory detail, etc.
Great for 4th and beyond! Students can use this on their own when reading over their papers of any length.
Bonus: 2nd page includes topics that are good for conferencing with students about.