"Just say a simple, "Yes, I will," or "No I won't." Anything more than this is from the evil one."
After reading some Bible commentaries, it seems that in Jesus day it was a custom to say, "I swear by my head that what I am saying is true or "I swear by my heaven I will do it."
Neither shalt thou swear by thy head - This was a common oath. The Gentiles also used this oath. To swear by the head was the same as to swear by the life; or to say, I will forfeit my life if what I say is not true. God is the Author of the life, and to swear by that, therefore, is the same as to swear by him.
Because thou canst not make one hair white or black - You have no control or right over your own life. You cannot even change one single hair. God has all that control; and it is therefore improper and profane to pledge what is God's gift and God's property; and it is the same as swearing by God himself.
Neither by heaven, etc. - It was a custom among the Scythians, when they wished to bind themselves in the most solemn manner, to swear by the king's throne; and if the king was at any time sick, they believed it was occasioned by some one's having taken the oath falsely. Herod. l. iv.
I remember as a child making oaths that I was telling the truth to my friends. Our favorite one was, "Cross my heart and hope to die, stick a needle in my eye!" I'm not sure why sometimes we didn't believe each other. I suppose a lot of lying was going on! Anyway, we need not use oaths to make a point.