Point out spelling commonalities both orally and visually. Help kids get the basics down pat before dealing with variations and exceptions. For example, you might spell out loud together words ending in "ee," chanting "bee b-e-e." "tree t-r-e-e, "see s-e-e" and so on. Type up some silly sentences and read through them with kids, providing them with a highlighter to mark all the double "e" words, both at the ends of and within words.
Bring up spelling in various situations. When checking out a book at the library, you might call your child over to point out that the title of the book is "Over the Sea," and that "s-e-a" is a variation of the "e-e" sound at the end of a word. When making a grocery list, you might say, "Now here's a spelling trick I learned. I need to buy dessert for tonight's dinner, but if I write it with only one "s," the word spells "desert," so I always think that the "s-s" in the middle stands for "strawberry shortcake."
Turn the weekly spelling test into a challenge; promise a reward for each night as soon as all the words for the week have been mastered. Rewards might include staying up an extra 10 minutes or playing a board game together. Help with the memorization by first eliminating any words the child already knows how to spell. Type up the remaining words and drill the child on as many as he chooses for each night. If he masters all the words the first night, the reward will be in effect all of the remaining nights---after the child quickly spells aloud (or writes, if he prefers) the entire list.
Make use of spelling lists to teach like-sounding words rather than like-meaning words. Kids will sometimes bring home spelling lists based on various subjects, such as animals. While helping them memorize the spelling of these words for their weekly test, note a couple of words to use at home in a list of words spelled alike. For example, if "dog" and "goat" are on the school spelling list, make a list of words that rhyme with each of these. You might place one of these lists on the dinner table and make up rhyming sentences together.
Practice Visualization. In her book "The Everything Kids' Spelling Book" (Adams Media 2009), Shelley Galloway Sabga suggests visualizing words as a method for improving spelling. For example, the word "balloon" is tricky because of the double letters; however, once a child can see two balloons floating away from their two strings, the word becomes more manageable. Another example: "fruit" is hard to spell because of the vowels; try writing it out and saying "u" will eat a banana, and "i" will eat an apple. Draw a giraffe with each leg an "f" and an "e" for a tail.