Match the electrode holder to the current you actually weld at. Up to 200 A a 200 A holder with a 25 square millimetre cable is enough, up to 300 A take a 300 A holder and 35 square millimetres, at 400 A and above a 400 or 500 A holder with 50 square millimetres. A cable that is too thin heats up, drops voltage and spoils the arc with longer basic electrodes.
Circuit length counts both ways. With 20 metres of welding cable and 20 metres of earth lead, the current travels 40 metres of copper, so you go one size up compared with working next to the machine. On site, coil spare cable in a figure of eight rather than a tight loop, because a tight coil acts as a choke and raises circuit inductance.
The earth clamp is the most often overlooked part and is responsible for half of all unstable arc problems. A spring clamp is fine for light work; above 250 A and for continuous work take a screw clamp with a copper jaw and fix it to clean metal, not to rust or paint.
Check the plugs. The DIX standard comes in 10 to 25, 35 to 50 and 70 to 95 square millimetre sizes, and the plug has to be crimped on the cable, not just pushed in. A loose connection heats up, burns out the socket in the machine and ends in a repair costing more than a set of cables.