Hardware problems can have more immediate serious ramifications than software problems because incorrect wiring can damage components. The most important tip is always disconnect power when making or changing connections, and double-check your work before connecting power.
Unplug Arduino from power while building and modifying circuits.
Applying power is the last thing you do to test a circuit, not the first.
For a complicated circuit, build it a bit at a time. Often a complicated circuit consists of a number of separate circuit elements, each connected to a pin on the Arduino. If this is the case, build one bit and test it, then the other bits, one at a time. If you can, test each element using a known working sketch such as one of the example sketches supplied with Arduino or on the Arduino Playground. It usually takes much less time getting a complex project working if you test each element separately.
For some of the techniques in this appendix, you will need a multimeter (any inexpensive digital meter that can read volts, current, and resistance should be suitable).
The most effective test is to carefully inspect the wiring and check that it matches the circuit you are trying to build. Take particular care that power connections are the correct way around and there are no short circuits, +5 volts accidentally connected to 0 volts, or legs of components touching where they should not. If you are unsure how much current a device ...