App.xaml

<Application x:Class="PhantomUI.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <Style x:Key="PlaceHolder" TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type TextBox}">
                        <Grid>
                            <TextBox Text="{Binding Path=Text,
                                                RelativeSource={RelativeSource TemplatedParent}, 
                                                Mode=TwoWay,
                                                UpdateSourceTrigger=PropertyChanged}"
                                 x:Name="textSource" 
                                 Background="Transparent" 
                                 Panel.ZIndex="2" />
                            <TextBox Text="{TemplateBinding Tag}" Background="{TemplateBinding Background}" Panel.ZIndex="1">
                                <TextBox.Style>
                                    <Style TargetType="{x:Type TextBox}">
                                        <Setter Property="Foreground" Value="Transparent"/>
                                        <Style.Triggers>
                                            <DataTrigger Binding="{Binding Path=Text, Source={x:Reference textSource}}" Value="">
                                                <Setter Property="Foreground" Value="LightGray"/>
                                            </DataTrigger>
                                        </Style.Triggers>
                                    </Style>
                                </TextBox.Style>
                            </TextBox>
                        </Grid>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsFocused" Value="True">
                                <Setter Property="FocusManager.FocusedElement" TargetName="textSource" Value="{Binding RelativeSource={RelativeSource Self}}" />
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Application.Resources>    
</Application>