A score of 50% on
ExamLab is good. ExamLab is one of the hardest mock exams I've seen. Giving the same mock exam over and over again isn't helpful as you remember most of the questions after 1 or 2 tries. Try to give different mock exams each time, that will give you better practice. Checkout the
ScjpMockTests page to get a list of free and commercial mock exams.
As for tips, don't stress too much, specially before and during the exam. If you have the K&B book, read the 2 minute drills 1 day before the exam...