Grab a box of flashcards - Flashcards can be such a great tool when helping children to practice and memorize their addition facts. Flash cards are especially good for those kids who don't have much trouble learning the information, but who hate to write. Some kids look at a worksheet and groan simply because they feel overwhelmed with the amount of writing that is involved. Using flashcards however, allows them to call out their addition facts quickly without any pressure to write.
Practice, practice, practice - It is not necessary to overwhelm a child with large amounts of work in order to help them to memorize their addition facts. However, it is essential that they have the material presented to them consistently. Don't add stress by rushing them to memorize their facts. Instead, introduce facts consistently so they will have plenty of time to practice.