I start the lesson getting the kids to think about the denominator and what it really means. I teach them the bigger the denominator, the small the piece that each person gets. Once my kids understand this concept, they can easily compare fractions with like denominators. The problem comes when the denominators are different.
I have my kids draw a butterfly shape with the fractions. This just helps them keep their numbers straight when they cross multiply.
Why does this work? We are finding equivalent fractions with like denominators. When the denominators are the same, you compare the numerators. Easy, right?