Graduate School in the Mathematical Sciences

Lots of opportunities are available to those with a bachelor's degree in mathematics. In some fields, such as biostatistics, finaincial mathematics, or operations research, a professional master's degree is preferred (or at least qualifies one for a higher salary). In research mathematics, a Ph.D. is required. Keep in mind: graduate school in the mathematical sciences is often free. Most Ph.D. programs in pure mathematics have financial support available in the form of tuition waivers plus a research stipend or a part-time teaching/grading job. This is also true for Ph.D. programs in statistics, applied mathematics, computer science, and operations research. Financial support for master's degrees varies wildly from field to field and sometimes from school to school; it's rarely available for pure mathematics, but is much more available for applied mathematics, statistics, financial mathematics, and biostatistics.

How should you go about applying to doctoral programs? Try this:

In September, register to take the GRE General Test and the Mathematics Subject Test. Yes, the registration deadlines are really early.
In October, prepare any fellowship applications you plan to submit. (The NSF Graduate Fellowship deadline is in late October. The NPSC and NDSEG deadlines are in December.) Provisionally decide on the graduate programs to which you'll apply.
In November, ask 3--4 professors for letters of recommendation. Start preparing graduate school applications.
In December/January, send applications to graduate programs including a safe school.
In March/April, visit all schools to which you're admitted and in which you're seriously interested.

Advice on selecting graduate programs: Apply to at least 6 schools, but no more than 12. (8--10 is safest here.) Include a program that it seems very likely will admit you. To help with gauging the appropriateness of various universities, get a ranking of programs (some are linked below) and ask some of your professors to mark a range of programs appropriate for you.

The Registry of Undergraduate Researchers might match you with a graduate school.
Want advanced work in actuarial science? Check the SOA list of programs and sort by level of courses offered, descending.
Make your own rankings for Ph.D. programs (that is, rankings based on criteria you choose). This site also has the most recent ranking of programs by the National Research Council as of 2010.
US News also ranks graduate and professional programs; check to see if your library has online access or the print edition. Also look for The Gourman Report, most recently published in 1997 but still reasonably relevant. Harvey Mudd maintains a list of graduate mathematics programs ordered by the National Research Council's 1995 rankings.
Here are hyperlinked lists of U.S. doctoral programs in mathematics and applied mathematics/operations research and statistics/biostatistics and mathematics education, so you can check out individual departments.