Students need to be taught common rules for spelling and then the exceptions to the rules. It is also important to evaluate how a student learns best. Will he or she learn best through a multi sensory approach? Writing in the air wihile visualizing and saying the letters aloud could I be the best method. Does listening repeatedly to a tape of spelling words said, spelled, and then said again helps cement information in the brain? Having your child make the tape in the first place can be highly effective. Categorizing different spellings for the same sound (for example, the long e sound can be spelled several ways) in a notebook to be reviewed on a regular basis will be extremely helpful.
I find that organization of the student's thoughts is the most effective way to improve writing skills. Using various graphic organizers to present ideas are very effective. The student fills in the graphic organizer and then expands their writing to complete the assignment. For instance, a student would put their main idea (my favorite season) in the center circle. Three or four circles attached to the circle would contain favorite aspects of the season (snow, holidays, my birthday). There are many types of graphic organizers that could be placed in a notebook.
If your child has difficulty expounding on words or phrases in writing, they could tape the information they want to present. Also, on computers, apps which enable speech to text could be used. The the child can get practice writing by rewriting the text.