Regular examination of your breast is a big step toward early detection. That's why all women over the age of 20 should examine their breasts once a month. 
You can learn this procedure from your doctor, nurse or clinic. You should perform this exam 10 days after the beginning of your menstrual cycle. You should do a thorough manual check.
Standing before a mirror or in the shower: With the right arm raised behind your head and using your left hand, knead the right breast in either a circular, up and down or wedge fashion. Repeat for the other side. You are looking for lumps or thickened tissue that feels different than the last time you checked.
Lying down: With the right arm raised behind your head and using your left hand, knead the right breast. Repeat on the other side. You are looking for lumps or thickened tissue that feels different than the last time you checked.
Next, look carefully in the mirror for visible changes. Squeeze your nipple gently to check for discharge. See your doctor immediately if you detect changes or lumps in your breast, even if they cause you no discomfort. A lump may only be a benign cyst, but your doctor must make that determination.
In addition to the self-exam, women between the ages of 20 and 40 should have a physical breast exam every three years. Women over the age of 40 should have a physical exam every year.