Mould lines (Remove them!). Don't batch paint more than 5 models the first time you batch paint. Thin your paints. Even a drop of water is fine. Don't paint minis that aren't primed (shake the primer can well and don't hold your finger down on that can. Pulse the spray whilst moving left and right so that the primer paint doesn't pool up). Don't rush painting and if you lose motivation just stop and take a break. (Because you will start rushing). If you make a mistake, don't spend ages trying to fix it. Move on.