I am a recent Mathematics grad who is taking courses in CS these days while I studying for the GRE mathematics test.
I have been studying for the math subject test for about 2 months using the Princeton Review book which is presumably available at any Barnes and Noble or Borders and is cheap on Amazon. Also, there's a book of exams "REA Best Test Prep" which provides many good practice problems.
The Princeton Review provides a skeletal outline of the most important topics to know for the exam, not necessarily an undergraduate mathematics education. In looking over the practice tests which directly come from ETS, I would say that 80-90% of the questions relate to the topics covered in the Princeton Review book.
When I first saw the ETS exams, approximately 75% of the questions intimidated my problem solving skills. Now the practice problems are becoming much easier to solve, so I would say a few months of practice is required to be prepared.
After I read through the workbook, I plan to review my textbooks and notes and then work the practice tests under simulated conditions.
So, that's my strategy. I'm just trying not to get too stressed about it before April.
I have a question though. Which CS departments have a Math Subject Test requirement? Cal Tech has a program in Control and Dynamical Systems which is math and CS intensive. I've thought about taking the CS or physics subject tests along with this particular one.