Open physics textbook on the subject of the resistance of materials. Classical theory argues that in a variety of ways to organize materials electric conductivity are different, but they all have something in common - a free charge carriers. Thus, changing the conductivity may primarily by changing the number of free charge carriers.
Remember that represent a semiconductor material, and how they shaped electrical resistance (conductivity). As is known, in semiconductor materials carriers are either electrons fled for some reason its orbit of the atom, or a "hole" left after leaving their place of electrons. Thus it obtained two types of charge carriers. Therefore, increasing the number of electrons that can increase the conductivity of the body. The main way this can be done, is to heat the body. By heating the semiconductor can be increased as the concentration of free electrons are pulled out of their seats by the heat, and "holes" left who left their now free electrons.
Please note that not all materials carry electrical conductivity of electrons. There are a number of substances which have a sufficiently good conductivity, comparable with the conductivity of metal ions through a substance. Take an ordinary incandescent light bulb and turn it into an electrical circuit, in which instead of a key, use the break with two contacts, lowered into a glass of distilled water. You will find that the lamp does not light up. This suggests that the water - is a dielectric. Now, do not turn off the lamp on the power supply, pour salt into the water. You will find that the brightness of the lamp increases, which indicates an increase in the conductivity of the water. The cause of this phenomenon lies in the fact that the dissolution of the salt in water latter becomes electrolyte salts for the atoms fall into sodium ions and chlorine, which provide conductivity material.
It should be noted that the conductivity of the metal can not enlarge or reduce the above-described methods, because you can not change the number of free electrons in the metal. The main way to change the conductivity of the metal - it's change the geometric dimensions. To increase the electrical conductivity of the metal can reduce the length of the metal conductor or increase the cross-sectional area. Thus it is possible to simply reduce the number of collisions of free electrons with the metal lattice sites, thereby decreasing its resistance.