How does the heating of a resistor have an effect on its resistance?
The resistance of a resistor is given by the formula:
Rt = Rref (1 + A*(T - Tref)), where Rref is the resistance at the reference temperature, usually taken as 20 degree Celsius. And A is the temperature coefficient of resistance of the material the resistor is made of.
The resistance of a resistor can either increase or decrease with the temperature based on the value of A. It is usually positive for metals.
To maintain the resistance as close as the reference resistance it is essential to keep the temperature as close to the reference temperature. The heat generated when current passes through the resistor should be allowed to escape. It may also be beneficial to turn off the current at regular intervals and use ways of cooling the circuit like fans.