Keep everything in writing, from day one

The habit that wins arguments is started before there is anything to argue about. Every agreement, every price, every change, every promise about dates: written down at the time, in an email or a message the other side received, however friendly the phone call was. Not because anyone is lying, but because two honest people remember the same conversation differently a year later, and the version that survives is the one on paper. Start on day one and it never feels awkward; start after the first disagreement and it reads as an accusation.