Given a StackPanel:
Apple
Banana
Following up on Sergey's suggestion, you can define and reuse a whole Style (with various property setters, including Margin) instead of just a Thickness object:
...
...
Note that the trick here is the use of Style Inheritance for the implicit style, inheriting from the style in some outer (probably merged from external XAML file) resource dictionary.
Sidenote:
At first, I naively tried to use the implicit style to set the Style property of the control to that outer Style resource (say defined with the key "MyStyle"):
which caused Visual Studio 2010 to shut down immediately with CATASTROPHIC FAILURE error (HRESULT: 0x8000FFFF (E_UNEXPECTED)), as described at https://connect.microsoft.com/VisualStudio/feedback/details/753211/xaml-editor-window-fails-with-catastrophic-failure-when-a-style-tries-to-set-style-property#