Tools
The first consideration in circuit bending properly is using the appropriate tools. A soldering iron is essential if you want to reproduce any results you achieve. For similar reasons, you'll want switches on hand to install to reproduce results. Screwdrivers of all shapes and sizes will likely be desired as some companies use very obscure bits. Power drills will make the constant screwing and unscrewing easier.
Contact Points
Body contact points are points on the circuit board that affect the sounds being produced. Finding these points is entirely a process of trial and error and poking around. While a noise is being made by the device, poke in various areas of the circuitry to see if a change occurs. Be careful about where you poke as some contact may damage the device. Once you've found an effective body contact point, add a button so you can remember where it is and use it again.
Crashing
Crashing is something that will inevitably happen to anyone who bends circuits frequently. It will not happen on every device, but it is a common occurrence. The only way to handle a device crashing while you're bending circuits is to restart the device. You may be able to add a hard reset button to make this process easier. Learn how to restore the device to factory defaults in case the crash is more permanent than a standard restart can handle.
Common Mods
The limit to the modifications you can make through circuit bending depends on the device being modified as circuit boards vary between devices. Even so, there are some mods that are common and easy to do. A voltage drop mod causes the same set of symptoms as having a low battery in a device. These symptoms can vary greatly between devices. Many circuit benders will install LED lights wherever they can to a device, though this process requires a multimeter to execute properly.