More generally, the slightly gauchely named "Street-Fighting Mathematics" http://streetfightingmath.com/ http://streetfightingmath.com/ http://web.mit.edu/6.

More generally, the slightly gauchely named "Street-Fighting Mathematics" http://streetfightingmath.com/ http://streetfightingmath.com/ http://web.mit.edu/6.055/ http://web.mit.edu/6.055/ * Divide and conquer * Abstraction * Symmetry and conservation * Proportional reasoning * Dimensional analysis * Easy cases (plugging simple values into complex formulas) * Lumping (discrete approximations of continuous functions) * Probabilistic reasoning * Springs (approximate complex systems as simple oscillators)