Camkierhi did a nice tutorial on one style of graffiti:
As for myself, I pretty much freehanded anything that didn't follow the above:
Many of those didn't turn out as well as I hoped, but with graffiti it is meant to look a bit rough anyway.
If you are doing lots of graffiti, like my wall there, you want to think about how each piece overlaps the others. Try to imagine someone coming along to add their tag and going over a previous one just to fit it in; no respect for the previous tagger.
Also, what I did was to search the internet and find a style that you like. Then do a few practice runs on a spare piece of plastic until you are happy.
I also planned out the whole wall, with all the styles and overlaps, just on a piece of paper with a pencil. The final piece varied a little from that, but not by enough to be worried about.
Hope some of that helps a little.