Graduate School in Mathematics or Statistics and the GRE Mathematics Subject Test
A similar, slightly older document with more good advice: advice and information about applying to graduate school in mathematics or statistics [pdf].
Follow the link below to the ETS website for general information about the GRE subject test in mathematics and a practice test you can download.
Click here for an old GRE practice exam [pdf]. This is the OLD math practice book (selected pages including practice exam). This isn't the "rescaled" test, so it is no longer distributed by ETS, but it's still the second-best practice exam for gaining experience.
Follow the link below to a free site for current and prospective math grad students, including posting of Q&A, advice and, perhaps most valuable, a self-reported listing of applicant profiles and admissions results.
Here is a link to some helpful information about the general GRE: http://www.onlinemastersprograms.org/exam-prep/gre/