Omnigraffle Tutorial
Below is a link to a couple of great little tutorial for learning the nuances of Omnigraffle. Just open the file in graffle and follow the instructions. The basic tutorial is, well, basic. Skip it unless you're a new user. The tool tutorial is really useful though. I'd suggest spending more than a few minutes practicing with the pen tool so that you can master bezier curves - an absolute essential for getting a professional look on your diagrams.
I would highly recommend this for new and experienced users alike. I had considered myself somewhat of an expert, but even I learned a few little tricks with this tutorial.
http://www.omnigroup.com/ftp/pub/software/MacOSX/Extras/OmniGraffle/OGWalkthroughs.dmg
Enjoy!
