But somehow i am not getting proper preparation and cant give the test.
I don't quite understand. Are you saying you have tried the exam several times already and failed? Or was it mock exams you failed? Or you don't feel confident enough?
If you think you don't understand the material well enough, the root cause I imagine, I would suggest getting another book to reinforce the topics. Much better to spend a small amount of money on another resource which will help your comprehension, than waste much more on an expensive exam voucher... Then write and deploy some code examples of your own and learn from your mistakes -- that's extremely valuable too.
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / AmazonAmazon UK )