I have no famialiarity with the CBA exam or Body of Knowledge (BOK) but have successfully done several other ASQ exams. Most important is a strong knowledge of the exam but i think that many people find the Quality Council ofIndiana (CQI) books very useful. I'd also suggest taking a number of practice exams in a timed enviornment. I'd especially recomend taking ASQ's practice questions which are retired from old exams -in they show the actual caliber of the questions which i find much better than other sources .
Best of luck,