You will need to adjust the validation patterns in the Value tab on the object palette. There are four patterns that you can play with (Display, Edit, Validation and Data). Each pattern will affect the field data at different times. To get access to the patterns click on the Validation Pattern...button. A new dialog will open up with the 4 patterns available to you. The display pattern is how the data will be shown on screen. The edit pattern is how the user will enter the data, the Validation pattern is how the field will be validated and the Data pattern is how the data will appear in the saved XML.
In your case you will want to set a disply pattern of DD/MM/YY and an edit pattern of DDMMYY. This will allow the user to enter 080708 or 08/07/08 and it will display 08/07/08 all of the time.
Hope that helps