Fake spray paint stencil method: first get yourself TiltShift generator (Adobe Air needed) - Link
First make a stencil, I made this Stanley Hudson using photoshop and saved as jpg. Then run it through the app, save and place over a background image (I set layer to multiply) and thats it! Its not the best method but it only takes a couple of mins and it's fun to play around with TiltShift.
You can run it through the TiltShift app multiple times to achieve that blurry, non-crisp spray paint edge in different areas of the "stencil".
Other tips: add the "slits" for the stencil islands and finish off with some drips and splatters.