Utilize flash cards. Write down words on cards and read them to the child. Have him spell the word back to you. If the word is misspelled, have him spell it again at least three times in a row correctly.
Instruct the child to write the word down on a piece of paper. Some students will have an easier time spelling a word correctly if they can work it out in front of them and see the letters as opposed to spelling it from direct memory.
Give the child stories or books so she can copy paragraphs onto paper to get her into the habit of both correct spelling and sentence structure. If it is a story she finds interesting, she will enjoy the exercise more and will likely take a more active role in participating.
Encourage the child to communicate with pen pals. Writing to a friend on a regular basis will help encourage proper spelling, especially if you assist in proofreading the message before it is sent and going over spelling errors with the child. Handwritten letters are more useful for this since mistakes won't be automatically fixed by the computer. If using a computer, turn spellcheck off until you are ready to proofread the message.
Use spelling kits designed for your child's age. There are various books and exercises specifically designed to improve writing, spelling and reading comprehension for kids of all ages, and they include interactive exercises you and your child can do together.