A case study I read recently showed positive results after a mother started proofreading her daughter's homework. She used specific symbols for different types of errors. This is an easy strategy that will only take a few minutes a day. You can ask your child's teacher for the proofreading marks used in the classroom.
Encouraging children to write for purposes outside of school, such as a family newsletter, provides additional writing practice, and an opportunity to revise the draft together. Perhaps word choice or organization need to be tweaked . Working through it together and adding photos and illustrations can be lots of fun.