The most basic of all rules is syntax -- word order -- because without it all language would be gibberish. An effective writing skill worth developing is using the active voice-- subject, verb, object -- because children or ESL learners find it easier to understand. For example, "Jon hit the ball," rather than "The ball was hit by Jon," makes more sense and is closer to spoken English. The easiest way to develop this skill is to have students write as they speak.
Grammar is the framework upon which language hangs. Like signposts, it tells the reader what is happening. One way to develop writing skills is to read widely from various genres: fiction, nonfiction, poetry, essays and letters. Students develop writing skills by reading, and they can learn the rules -- almost by osmosis -- by being exposed to a diverse variety of literature. Learning formal grammar rules is tedious and quickly forgotten, while actively using them to improve writing skills makes it a "real" experience that can be incorporated into writing immediately.
Another rule students have to learn when they write is that the subject -- the noun -- has to agree with the verb. Understanding how to conjugate verbs in the past tense is an essential writing skill: He was, is and will be. An effective habit to develop is reading the work aloud and listening for mistakes that students will hear, but not see.
Once students have the basic writing skills and can construct sentences that make sense, it is time to move on to "dressing up" their writing by introducing adjectives -- words that describe nouns -- and adverbs, words that supplement verbs. "Blond haired, brown-eyed Harry quickly ran home as fast as he could" paints more of a picture than "Harry ran home."
The Teaching Writing Skills website advises introducing new vocabulary words and having students incorporate them into their writing to expand their writing skills. One idea is to keep a vocabulary book and write sentences with new words to better understand them in context. Using a dictionary and a thesaurus also improves writing skills.