Another tip is do not stare too intently at where your brush is on point. Like walking with a full cup of hot coffee, look straight ahead not at the sloshing liquid inside or it'll be splattering the floor in front of you. Same basic idea here with the painting. Watch where you're going a bit ahead of the brush. Staring directly at it has only led me to wobble and bobble it.
Once that happens, you will have a little bobsled action, the brush ricocheting of one side of the narrow lane then the other as it travels down the track. Your line will wind up rippled and wavy. You will then begin the process of covering it up which inevitably leads to a cycle of swerving fixes which leads to terrible frustration which leads to giving up and painting your piece all the same color. Like I did today. Eh, it happens.