Will the GRE Math Subject Test Really Help Your Application?
The first thing I want to state is that the best way graduate schools assess sufficient math background is through traditional coursework. So you will have no other great option to slogging through a bunch of very tedious lectures. Although, a masters especially from programs like LSE would come in handy, it would be more feasible and even give you an edge to just take some classes as a non-degree student at a reputed university.
I wanted to get that out of the way before digging deeper into this question which is probably on a lot of aspiring economists/finance gurus minds. The thing is that it would be much better to take the actual courses when they are required. And it is going to be a poor way to show mathematical ability if you don't have a good undergraduate math background since it is such an intensive test that requires a lot of preparation.
Mathematicians for sure need it, but how useful would it be for others to invest the time and money into taking and acing a very challenging test. People might disagree with this, but a good score on the GRE Math Subject test can offset in some instances any bad grades in requisite undergraduate math courses. Still, the math subject test requires mathematical acumen built up over years of studying math. And it is understood that the subject test is no real substitute for taking the actual courses. So only choose the GRE Math Subject Test if you absolutely have to i.e., lack of finances, not enough time, etc.
The GRE Math Subject test can really work in your favor when you otherwise have a stellar profile when it comes to the research you conducted or your GPA and the challenging courses you took (minus the math ones of course). Moreover, it is nice to have a great GRE Math Subject Test score when applying for certain graduate programs such as physics and computer science among others.
How to ace the GRE Math Subject Test
Real analysis is tested and so is undergraduate level linear algebra and differential equations.
Although, the practice tests are great to have, the Princeton Review and Kaplan books aren't as useful for those aiming at high scores above 800 which is actually in the 86-88th percentile since subject tests have a maximum score of 900. If you want to score in the 90th percent - a great score - you'll have to be a calculus whiz and be able to do Calculus I-III problems really fast. If you can do this then the topology, number theory and miscellaneous sections will be a breeze.
A better approach would be to work on as many problems from Stewart Calculus in addition to focusing on individual areas through books like Rudin Real Analysis and Insel Linear Algebra. If you haven't done much since Calculus II, then it might be helpful to sign up for a class or two at a nearby community college or even a state school. You could even do summer school and take differential equations or multivariable calculus at a prestigious institution like Harvard. It's nice to be challenged and have a rigorous schedule to follow while competing for a good grade with other students in an academic setting. An "A" grade from Harvard would sure look good in addition to prepping you for the GRE Math Subject Test.
Basically, a 90 percent plus percentile will make your profile really stand out to the admissions committee. I hope this post helped solve your questions and put you in the right direction when it comes to acing the GRE Math Subject Test if you opt to take it.