+8 on using USMLEWorld. Better questions and cheaper than Qbank.
Overall strategy: I took about 4 weeks to study for Step II. I did not study nearly as hard core as I did for step one, but I still studied 4-6 hours a day most days for 4 weeks. I did this during an easy elective that involved a couple hours of class room time a couple days a week.
First I read through the Kaplan books (I choose them because I got them for free-- they are good, but I think blue prints or First Aid would be just as good). Regardless, I just did a general overview for most of the first week. Then, I did questions...
Do all the questions you can. Do all of USMLEWorld. All of it. Find more questions and do them. I split a Qbook with a friend and had another 400 or so high quality questions. I also got a week's worth of QBank from a friend (he had taken his test and still had a week of subscription left) and did another 500+ questions. I found the books from my weaker shelf exams and did the questions in the back of them.
Anyway, this question-based strategy worked wonders for me... my step one score was nice but not overwhelming, and I raised it by about 30 points on step II. Whenever you get tired of questions, read over things you have identified as weaknesses, say, pediatric developmental milestones.
The fact that you are getting 30-40%ile on the shelf exams isn't wonderful; try to discover your weakness(es) in studying for them and rectify that for your step II studying. You did well on Step I so you know what it takes.
As for when to take it, I took it in late October and my scores were in the night of my first interview. If you are counting on a higher score just to get interviews, I might take it earlier. If you think you can score interviews and want a higher score to help you match/get a couple extra late interviews, late october worked for me.
Good luck. Don't develop tendonitis doing all the questions.