LEB Hobbies Stainless Steel Braid Brush with plastic handle to keep braids clean, thin and oxide free.
The key to optimal performance is good electrical contact. Dirt and oxide can accumulate on the braids of a car, which will interfere with the electric signals reaching the car properly. Brush the braids with a non-residual solvent like lighter fluid (naptha) or isopropynol alchohol. Make sure the flammable solvents have completely evaporated before running the car on the track.
Braids should also not be allowed to get "bushy" since stray wires of the braid can hold the rest of the braid away from the rails. Brushing the braids so they are kept as thin strips will also help with connectivity.