I use 4 conductor No. 14 600 volt control cable. You can't find it in Home Depot, but if you know any electrical contractors you might get them to get some for you at less than $0.50 per foot.
By paralleling two wires of the cable, the impedance, which increases considerable with frequency, is cut in half. Increasing wire size reduces the resistance, but doesn't reduce the total impedance nearly as much. Using four wires instead of two cuts the impedance about in half. For instance, at 20 kHz, the impedance of 50 feet of 2 conductor No. 10 (2.5 times as large as No. 14) is 1.10 ohm. The resistance is 0.17 ohm.
The total impedance of 50 feet of 4 conductor No. 14 is only 0.58 ohm, and the resistance is 0.31 ohm.
An added advantage is the ease of bi-wiring if the speakers accept it (as do my RF-3 and RC-3 speakers).
For shorter lengths, say 10 feet or less, the impedance really doesn't matter much because it is much smaller than the speaker impedance.