The best way to reheat sushi rice so it doesn't dry out is to put it in a microwave-safe bowl and place an ice cube in the center with a piece of parchment paper over the top (see my salmon rice bowl recipe for photos). Reheat it for 1 minute and 30 seconds then remove it from the microwave and discard the remaining ice cube. This method adds moisture back into the rice by gently steaming it.