I found this answer in Quora which has some very useful documents, videos and articles. Both answers are really good:
http://www.quora.com/Job-Interviews/How-should-I-prepare-system-design-questions-for-Google-Facebook-Interview
Also:
How to Rock a Systems Design Interview
How do I design an arbitrary system in an interview?
System design – This is a very complete step-by-step guide/tutorial
System Design Tutorials – A collection of commonly asked System Design Interview Questions with Solutions