That is a very long run......
1) Choose a wire at most 8 gauge, not any more. Basically the idea is to reduce the resistance. The thicker the wire (lower the gauge), the less its resistance. The resistive contribution of the wire should be no more than about 5-10% of the output impedance of your amplifier, to minimize losses.
2) Make sure it is copper as pure as it can be. Silver is a bit better but at that length it will cost a ridiculous amount.
3) Do NOT coil the wire, keep a straight line run as much as possible, otherwise inductance will increase.
4) Try and change your room as a last resort, that long a wire is very unusual. I am sure your speakers are not 120 feet apart....