WPF Combobox DefaultValue (Please Select)

All good answers that has been supplied, but I used the following to solve my problem

<ComboBox SelectedIndex="0">
    <ComboBox.ItemsSource>
        <CompositeCollection>
            <ListBoxItem>Please Select</ListBoxItem>
            <CollectionContainer Collection="{Binding Source={StaticResource YOURDATASOURCE}}" />
        </CompositeCollection>
    </ComboBox.ItemsSource>
</ComboBox>

Thanks for everyone who has helped!

Add these properties to your combobox and you can set a default 'Please Select' Text on a combobox.

<ComboBox IsEditable="True" IsReadOnly="True" Text="Please Select"/>

For a more versatile solution you can create a watermark for the combobox

I did this with mine, works for me, since I have static items.

<ComboBox Name="cbxType" HorizontalAlignment="Left" Margin="116,41,0,0" VerticalAlignment="Top" Width="192">
    <ComboBoxItem Name="create" IsSelected="True">create database</ComboBoxItem>
    <ComboBoxItem Name="update">update database</ComboBoxItem>
</ComboBox>