The Stencil Revolution is not only a great site for looking at examples of detailed and beautifuly executed graffiti (take a look at this one), but it now hosts a great series of tutorials.

These tutorials explain how to make t-shirts, multi-colored stencils, and stencils from photoshop images.

...and especially good for us teachers busy instilling law-abiding social activism: non-permenant chalk stencils.

This site also has some great downloadable fonts that have all the connecting bits to work well in your students' stencil designs.

The students in my class love doing stencils. They are constantly working on them inbetween (and occasionally during) other projects. One of the pillars in my classroom is an ongoing stenciling project. If you can provide a space for this in your classroom, its an excellent outlet for students who are looking to leave their mark.

...and then they use those stencils to make chalk art elsewhere...

Here's another stencil resource if you are looking for more.

Update: Stencilry has a cheap, quick, and dirty stenciling technique to show you.