You can use the Ctrl + ' combination, you can use a Function or you can use something like this (I have used a field called FirstName. Replce it with the actual name of your control.):
In the After Update event put this code:
Private Sub FirstName_AfterUpdate()
Me![FirstName].Tag = Me![FirstName].Value
End Sub
In the On Enter event put this code:
Private Sub FirstName_Enter()
If Not Me.NewRecord Then Exit Sub
If Not (IsNull(Me![FirstName].Tag) Or Me![FirstName].Tag = "") Then
Me![FirstName].Value = Me![FirstName].Tag
End If
End Sub