Ash Wednesday is similar to Transfiguration in that the themes associated with the day through the texts, the history of the day and liturgical practice are so many it is hard to concentrate on a one line of thought for preaching.
One particular line of reflection is that of "return". "Return to me," says the Lord through the prophet, Joel. The words spoken when ashes are imposed ("remember that you are dust and to dust you shall return") are the words of exile. They are banished from life itself, God, on account of sin. Our first parents were also exiled from the garden and the way was barred. In Christ God is calling us back. Christ is the way.
That is the call of Lent. Return. We return with repentance: fasting, praying, giving of alms, that is, we return in Christ. Lent, historically, was a time of return for penitents who were literally put of the church and on Ash Wednesday and through the practice of repentance, that is, faith were reconciled on Maundy Thursday and joined with the faithful in the Great Vigil of Easter.