Hi, BITCHES!
Let's start a thread discussing techniques etc. Hopefully this will become a rich resource which people will add to and hopefully it will get stickied..
ShortingRandomly connecting points on the board. Straight solder point bending. Make sure you don't fry components by connecting them to high voltage points.
Component replacementExample: replacing a resistor which controls pitch with a potentiometer. Doing this sort of thing with a pitch resistor will require a high resistance potentiometer, like several megaohms, to get a low pitch. Raising the pitch to high (no resistance) may cause the machine to glitch. Another thing you could replace would be an oscillator, like the coils in VTech toys. You could create a 555 timer circuit to replace the coil and make your own signals to under or over clock it. This may affect the pitch. For more information see this reference:
http://www.sailormouth.org/index2.php?pg=addingosc.incLine cuttingExample: cutting integrated wires/lines on circuit board of keyboard between synth chip and CPU chip. Can lead to some interesting results, such as decimation of sound or video etc. A resistor may be used if the machine is analog or if this affects the machine in some way. Another example of use would be in a game console between the cartridge and the video chip, or between the rom or synth chip and CPU of a speaking toy.
Voltage drainPlacing a potentiometer or resistor between a power lead or line to a chip/machine. Can produce glitchy, strange sounds or glitches depending on the machine. May not be effective on every machine.
Touch contactsSome machines will have sensitive points which can be touched to short. An example of this would be a pitch resistor. Simply install some screws with solder tags into the case and solder them to the points to use.