The principle by which an endocrine system regulates its own secretory activity is known as negative feedback.
Negative feedback decreases the deviation from an ideal normal value, and is important in maintaining homeostasis. Most endocrine glands are under the control of negative feedback mechanisms.
Negative feedback mechanisms act like a thermostat in the home. As the temperature rises (deviation from the ideal normal value), the thermostat detects the change and triggers the air-conditioning to turn on and cool the house. Once the temperature reaches its thermostat setting (ideal normal value), the air conditioning turns off.
For example, your pancreas (an important gland in your endocrine system) relies on negative feedback to regulate blood glucose levels. An influx of glucose, say from a carbohydrate-heavy dinner, triggers your pancreas to produce a hormone called insulin. Insulin's message to your body is to take up that extra sugar into cells in order to bring your blood sugar back to the target level. Once enough glucose has been taken up by your cells, your pancreas stops secreting insulin.