Rant: Quarter panel weld warped on me last job

I had a quarter panel replacement job that went south fast. The metal warped bad after I finished the welds. I think I had the heat too high, but the customer was breathing down my neck for speed. Now the panel looks wavy, and I have to redo the whole thing. It's eating into my profit and time. Any tips on keeping sheet metal flat during welding? Especially under time pressure?