10 Ways NOT TO Kill An Arduino
method1: use a BJT switch made of transistor (such as 2N2222, ULN2003A. etc) to switch "high load" (e.g load exceeding arduino's rating). more here: http://whites.sdsmt.edu/classes/ee320/notes/320Lecture24.pdf
method2: use a Pull Up Resistor (or pull down resistor, which ever convenien) on Input pins. More here: https://learn.sparkfun.com/tutorials/pull-up-resistors/what-is-a-pull-up-resistor
method3: always check POLARITY of Supply
method4: Arduino is DC. IF want to control AC or higher load DC than the ones recommended for Arduino, use a SSR (Solid State Relay) or Mechanical Relay. More here: http://shin-ajaran.blogspot.sg/2011/10/arduino-solid-state-relay-ssr.html
method5: always check input (and also output) rating (e.g voltage, current, polarity) of the electronic components to used with arduino.
method6: check for common ground connectivity for all electronic components
method7: always TURN OFF supply to an arduino when connecting electronic compoents to arduino.
method8: use a diode (e.g IN4148) to ensure electrical signal (e.g current) only flow one direction
method9: use a logic level shifter connecting devices with different logic levels e.g 3.3v, 5v and 15v.
method10: always check circuitry and check again before turning on.