Given a StackPanel:
Apple
Banana
Another nice approach can be seen here: http://blogs.microsoft.co.il/blogs/eladkatz/archive/2011/05/29/what-is-the-easiest-way-to-set-spacing-between-items-in-stackpanel.aspx Link is broken -> this is webarchive of this link.
It shows how to create an attached behavior, so that syntax like this would work:
This is the easiest & fastest way to set Margin to several children of a panel, even if they are not of the same type. (I.e. Buttons, TextBoxes, ComboBoxes, etc.)