The Goodfellas over at Fakeproject have made a coloring book that's, a downloadable .pdf, to help you develop your skills. FCOA's statement is below.
----In a bid to increase the quality and quantity of graffiti, The Fakeproject Corporation of America has issued a coloring book, free of charge, which allows graffiti artists to practice their tags before application in a real world situation.----
The file has 18 different mediums to practice on from freights, to vans to phone booths to ice boxes.
Printable PDF format here.