i found this on the BBC website:
Ash Wednesday services
The service draws on the ancient Biblical traditions of covering one's head with ashes, wearing sackcloth, and fasting.
The mark of ashes
In Ash Wednesday services churchgoers are marked on the forehead with a cross of ashes as a sign of penitence and mortality.
The use of ashes, made by burning palm crosses from the previous Palm Sunday, is very symbolic.
God our Father, you create us from the dust of the earth.
Grant that these ashes may be for us a sign of our penitence, and a symbol of our mortality.
Traditional Ash Wednesday prayer
Priest anointing a member of the congregation with ashes from a large bowl Anointing with ashes ©
The minister or priest marks each worshipper on the forehead, and says remember you are dust and unto dust you shall return, or a similar phrase based on God's sentence on Adam in Genesis 3:19.
The modern practice in Roman Catholic churches nowadays, as the ashes are being administered, is for the priest to say something like Turn away from sin and believe the gospel.
Keeping the mark
At some churches the worshippers leave with the mark still on their forehead so that they carry the sign of the cross out into the world.
At other churches the service ends with the ashes being washed off as a sign that the participants have been cleansed of their sins.
Symbolism of the ashes
The marking of their forehead with a cross made of ashes reminds each churchgoer that:
* Death comes to everyone
* They should be sad for their sins
* They must change themselves for the better
* God made the first human being by breathing life into dust, and without God, human beings are nothing more than dust and ashes
The shape of the mark and the words used are symbolic in other ways:
* The cross is a reminder of the mark of the cross made at baptism
* The phrase often used when the ashes are administered reminds Christians of the doctrine of original sin
* The cross of ashes may symbolise the way Christ's sacrifice on the cross as atonement for sin replaces the Old Testament tradition of making burnt offerings to atone for sin