When I was a teenager, I used to use these kinds of tricks to do well on tests. Especially useful for multiple choices but works very well otherwise. The course material is inspiring and well organized. I highly recommend to take a look.

http://ocw.mit.edu/OcwWeb/Mathematics/18-098January--IAP--2008/CourseHome/index.htm

Sanjoy Mahajan, course materials for 18.098 / 6.099 Street-Fighting Mathematics, IAP 2008. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [5 Aug 2008].