I just had to look this up for a friend, who accidentally completely closed them.
The recommended way, taken from "Zinn and the art of Mountain Bike Maintenance" is as follows.
"Sometimes the pads in hydraulic disc brakes can rub because the pistons get pushed out too far, especially if the lever is applied without a rotor or spacer between the pads. You will have to push the pistons back in, usually by removing the pads and pushing the pistons back in with a plastic tire lever. On Hayes, the only thing pulling the pistons back in is the reversal of the twist the pistons apply to square-cross-section o-ring seals surrounding the waist of each piston, so the pistons tend to stay out too far once there. Pull out the pads, carefully push the pistons back in with the box end of a 10mm wrench(avoid pressing on the pin sticking out of the piston, which hooks the wire catch on the back of the pad), and replace the pad"