What is the easy way to set spacing between items in StackPanel?

if all the controls are the same then do as IanR suggested and implement a Style that catches that control. if it's not then you can't create a default style to a base class because it just won't work.

the best way for situations like these is to use a very neat trick - attached properties (aka Behaviors in WPF4)

you can create a class that has an attached property, like so:

public class MarginSetter
{
    public static Thickness GetMargin(DependencyObject obj)
    {
        return (Thickness)obj.GetValue(MarginProperty);
    }

    public static void SetMargin(DependencyObject obj, Thickness value)
    {
        obj.SetValue(MarginProperty, value);
    }

    // Using a DependencyProperty as the backing store for Margin.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty MarginProperty =
        DependencyProperty.RegisterAttached("Margin", typeof(Thickness), typeof(MarginSetter), new UIPropertyMetadata(new Thickness(), CreateThicknesForChildren));

    public static void CreateThicknesForChildren(object sender, DependencyPropertyChangedEventArgs e)
    {
        var panel = sender as Panel;

        if (panel == null) return;

        foreach (var child in panel.Children)
        {
            var fe = child as FrameworkElement;

            if (fe == null) continue;

            fe.Margin = MarginSetter.GetMargin(panel);
        }
    }


}

now, to use it, all you need to do is to attach this attached property to any panel you want, like so:

<StackPanel local:MarginSetter.Margin="10">
    <Button Content="hello " />
    <Button Content="hello " />
    <Button Content="hello " />
    <Button Content="hello " />
</StackPanel>

Completely reusable of course.

I use a transparent separator, which works well:

<Separator Opacity="0" Height="20"/>

You can of course use margins but then if you want to change the margins you have to update all of the elements.

The separator can even be styled in a static resource.

An attached property could do it too but I think it's overkill.