Place the sheet metal on sand and leave it there as you weld. The sand absorbs heat from the welding and helps the metal cool more rapidly after you finish.
Clamp the two pieces of metal together, before welding. This ensures that, even if they warp, they warp together. Situate the two pieces of metal slightly away from their intended post-weld position, so that when they warp, they warp into position. If you cannot predict the direction in which your metal will warp, consult a welding professional.
Create a minimal number of small welds, at regular intervals to each other, on your sheet metal. Fewer and smaller welds translate into less heat-affected zones and less warping. Make as few welding passes as possible. Avoid having to create a second set of small welds, by achieving sufficient welding integrity on the first pass.