"You are the light of the world. A city on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house. In the same way, let your light shine before men, that they may see your good deeds and praise your Father in Heaven."
I love the way John White puts it:
"Has it never dawned upon you that the essence of witnessing is just plain honesty? You are salt - whether you feel like it or not. You are not told to act like salt but to be what you are. You are a light. God has done a work in your life. Don't try to shine. Let the light that God put there shine out. It demands no more than honesty. It demands honesty before unbelievers. In fact such honesty is ninety per cent of witnessing. Witnessing is not putting on a Christian front as to convince prospective customers. Witnessing is just being honest, that is, being true to what God has made you in your speech and everyday behaviour." John White