My mom is a breast cancer survivor, and she found her lump herself during an exam. She always said the best time to do the exam is in the shower, when your skin is wet, because it's easier for the fingers to glide over the skin.
She also advocates the "breast as a clock" method, where the woman starts at 12 o'clock and feels all around the breast until she's back at 12, moving in concentric circles inward or outward. That way, you cover the whole breast. It's also a good idea to check under the arms, too, since a swollen lymph node there can be a concern.