Here is a great quote about Ash Wednesday from Lauren Winner’s Girl Meets God.
The imposition of ashes on Ash Wednesday is nothing if not bold – a dark and undeniable slash across your forehead, a bold proclamation of death and resurrection all at once. You forget that it is on your forehead and you walk out of church, out into the world, a living reminder that Christ died for us… The cross on our foreheads is meant to be a dramatic reminder to ourselves – and it is that. When Milind looks at me and says, “Remember that you are dust and to dust, you shall return,” I know what God did for me. He not only created me, he then poured out his grace upon me in the blood of his son. Me, a bunch of dust! But the cross also stimulates other people’s questions. It provides an unmistakable opportunity – even obligation – to witness.