Raise awareness. Go to www.wm3.org and read about the case. They have T-shirts, bracelets, dog tags, and bumper stickers for sale that help fund the defense fund. They also announce various benefit events happening around the country. They also have a paypal account where you can simply donate to the defense.
You can discuss the case with other supporters and some non-supporters at the West Memphis Three Discussion Board.
Several Music Artists have contributed to Compilations to benefit Damien, Jesse, and Jason. The Metallica track does not benefit the defense, but Damien Mentions it in Paradise Lost: The Child Murders at Robin Hood Hills and I think of him every time I hear it.