The basic how-to: For poultry, insert the thermometer into the inner thigh area, near the breast but not touching the bone (bone is a better conductor of heat than meat, so the reading will be higher than it would be for meat farther from the bone). For beef, pork, lamb, veal and ham, aim for the thickest part, staying away from bones and fat. Here are the USDA's recommended safe minimum internal temperatures for meat.
Common mistake: Not calibrating. A thermometer can become inaccurate over time, so reset it by putting the thermometer in a glass of ice water and waiting until the indicator stops moving. With the probe still in the water, use a wrench to turn the thermometer's adjusting nut until the dial reads 32 degrees.