If you happen to be a person with a strong visual memory, you remember how words look. You write a word and take a look at it. You say, That looks funny. Let me try it a different way. Poor spellers, however, can't recall how a word should look.
In How to Teach Any Child to Spell, Gayle Graham offers a simple solution to the problem: Pull the student's own misspelled words from the context of his own writing.