A power transistor is very sensitive to heat, making it prone to heat destruction. When the transistor is carrying large amounts of current, it heats up. If steps are not taken, it will self-destruct. The only way to counteract the heat is to mount the transistors on top of a heat sink. A heat sink is a ribbed aluminum plate, which dissipates heat away from the transistor and into the air. Some very high power applications may even require a cooling fan blowing on the transistor to reduce heat.
You have to size up the wire gauge to the wattage the transistor is carrying. The more the wattage, the thicker the wire. Wires are rated in terms of ampere-carrying capability. For example, a 14-gauge wire can handle 15 amps of current. Most electronic circuitry is very low power, so the wires are thin. Do not presume all parts of a circuit are low power. In the engineering drawings, the designer of the circuit specifies the size of the wires in the power output section. Follow the specifications closely, since using wires that are too thin will eventually cause the wires to overheat and possibly catching fire.
Electronics designers at "Circuits Today" recommend a well-regulated and filtered power supply. Transistors are sensitive to voltage fluctuations, especially voltage spikes that burn out the transistor. You have to ensure a smooth voltage to the transistor. Also, filtering is a requirement. Transistors are not designed for a waveform that is full of static. If a constantly "raspy" wave is fed in, eventually the lifespan of the transistor is shortened. Electronics technicians constantly replace power transistors that have failed. One of the chief reasons is voltage spikes or static.
When you are designing a circuit, look to the engineering specifications for the transistor you are using. Do not exceed the current rating of the transistor. If your circuit calls for currents outside of the design limits, look to find another transistor. Also, think about the frequency response. Many power transistors are designed to work in the audio range, but not in the microwave range. If you need a high-frequency transistor, look to the engineering specifications for one. You almost have to work backwards, starting with the engineering sheets first. Look for a transistor that meets your needs and then design the circuit.