I'm compiling a list of resources as I prepare; I will also be taking the test in April. Here's what I have so far.
1. There is a practice test online. I think this is the same booklet they mail you. Become familiar with all the ETS info. Like the subject distribution (50% calculus, 25% algebra, 25% other).
2. Two preparation books are currently in print:
0375764917 Cracking the GRE Math Test
0878916377 GRE Mathematics (REA)
The first is considered the better of the two according to comments on Amazon.com. I've been working out of the first book so far. There is also a book by Morris Bramson no longer in print containing five or six actual exams. If you know where to find this, let me know.
0668056754 Mathematics: Subject Test/Advanced
This web page has some good info on the test:
http://math.scu.edu/~eschaefe/gre.html
I have a feeling the problem type distributions were calculated from Bramson's book.
These books as well as the sample exam available for download will help you key in on the type and level of difficulty of the problems found on the exam. They will also highlight your deficiencies and guide your study plan.
3. Use what you have. After going through the resources in [2], go back to textbooks to fill in spots and practice the basics. Then, try the harder problems you may have skipped when you first went through the class. Stewart's Calculus, for example, has "Problems Plus" at the end of each chapter that has problems of the type and difficulty level found on the GRE exam.
4. Once you exhaust the resources you have, begin searching for others. Purchase texts in all the standard fields (algebra, analysis, topology). Also look into problem collections. You must be confident in your grasp of theory and computations.
I just bought two of the Schaum's outlines, "Advanced Calculus" and "3,000 Solved Problems in Calculus". Once I work through these, I'll be able to give a more thorough review (with GRE prep in mind). In another thread I was recommended "Exercises in Algebra" by A.I. Kostrikin. I can't speak to this book, either.
5. GRE preparation should invariably lead to preparation for PhD Entrance Exams. Many universities have past exams available for download. Problems are more theoretical in nature than computational but will prove you have a true mastery of the subject.
0387949348 Berkeley Problems in Mathematics
http://math.berkeley.edu/~desouza/pb.htmlOk, that was a sort of coherent outline of my approach and the resources I've bookmarked. Now get off the internet and start studying!