Write out words that they are having trouble spelling and leave the letters that are giving them trouble blank. Have them fill in the missing letters. For example, if they have written becuase, write bec se and have them fill in the blanks. This shows them the part of the word they are having trouble with and helps them learn that part.
Highlight tricky letters in a word.
Write problem letters in a different colour.
If they are having trouble with a particular word, have them write down several ways of spelling it and ask them to pick the way that looks right to them. Usually they are right.
Have them sort words by visual pattern. Like words that drop the final e when adding-ing (hoping, joking, caring, hiking).
Make a word wall on the fridge or in a notebook that they can reference for writing. Add words that give them trouble so they can look to the wall when they can't remember the spelling of a word.
Remember to talk to your child's teacher about the words used in class and use age-appropriate words that your child will recognize.