Techniques and basics

Lettering
http://www.graffitistudio.net/ultimate-guide-draw-beautiful-graffiti-letters

Six steps to becoming an artist
http://www.graffitistudio.net/steps-become-graffiti-artist

10 steps to making a stencil
http://www.wikihow.com/Make-a-Graffiti-Stencil

Tutorials good!! Everything you might wanna know!
http://www.stencilrevolution.com/tutorials/

how to draw different styles
http://www.dragoart.com/graffiti-c381-1.htm

Creating a Tag
http://graffitidiplomacy.com/CreateYourOwnTag.html

Tipos and exercises to improve your spraying (good)
http://abduzeedo.com/tips-and-exercises-master-spray-paint-0