A prayer Richard Foster uses when beginning a time of contemplation:

By the authority of Almighty God,

I surround myself with the light of Christ,

I cover myself with the blood of Christ,

and I seal myself with the cross of Christ.

All dark and evil spirits must now leave.

No influence is allowed to come near to me

but that it is first filtered

through the light of Jesus Christ,

in whose name I pray,

Amen.

This is taken from Richard Foster's book, Prayer: Finding the Heart's True Home.