Break or chop large pieces of chocolate so it will melt more evenly. Melt chocolate in a dry heavy saucepan over low heat; stir until smooth. Even small amounts of water will cause the chocolate to seize (become thick and lumpy) and become unusable. If the chocolate needs to set up after melting (such as when it's used for dipping or garnished), add 1/4 to 1/2 teaspoon of shortening for every 6 ounces of chocolate.
To melt chocolate in the microwave, place it in a microwave-safe bowl. Melt semisweet chocolate on high (100% power) for 1 minute; stir. Microwave at additional 10- to 20-second intervals, stirring until smooth. Melt milk chocolate and vanilla or white chocolate at 70% power. Stir frequently until the chocolate is melted; do not overheat.