I bought my voucher with the free retake option a year ago with the intent of starting to study as soon as the SCDJWS book from Mcgraw Hill that I pre-ordered came out. The book never came & I got busy and forgot about it. About 2 months before the voucher expired I realized I had to get off my rear end and study! My studies consisted of using the Whizlabs Suite and Mikalai Zaikin's study guide. I had to exercise my free retake, but I was able to squeak by on the day before my voucher expired.
Mikalai Zaikin's study guide was extrememly helpful as was the Whizlabs exam simulator. I'd also suggest downloading the WSI Basic profile and becoming one with it. Most importantly don't put off studying!