Item spacing in WPF ItemsControl

I'd add an ItemTemplate where you set the margin

<ItemsControl.ItemTemplate>
    <DataTemplate>
        <TextBlock Margin="3,3,3,3" Text="{Binding}"/>
   </DataTemplate>
</ItemsControl.ItemTemplate>

Provide style to your ItemsControl containers (default ContentPresenter) like this where you can set Margin to say 5:

    <ItemsControl>
        <ItemsControl.ItemContainerStyle>
            <Style>
                <Setter Property="FrameworkElement.Margin" Value="5"/>
            </Style>
        </ItemsControl.ItemContainerStyle>
    </ItemsControl>