I have passed my exam today with 91%. To get this certification I have:
* read the K & B study guide (i was also studying for a MSc so i finished reading this yesterday hehe). Give yourself more time say 3 months to read this book and take every word in.
* Using the whizlabs website and Enthuware test studio (these are difficult questions and my average was only 65 in these exams)
* Write plenty of code (which im sure you are doing)
For me, I didn't get any questions on Inner classes, serialization and wait/notify. The exam focused on access modifiers, generics, variable scope, asserts, casting, method overriding, NumberFormat.
The experience of preparing is essential and a main tip I would say is to write code on as much of the concepts as possible to fully understand them for now and they will stay with you throughout your career.
Im a constant reader (first time poster) from this site so i wanted to share my experience and I hope this helps. Feel free to ask for any general questions regarding your prep.