Washing the Coat
Washing tends to make a plastic coat soften. Before washing read the washing instructions of the coat carefully. If you find it machine washable then wash it with a mild detergent in cold water and hang it so it can dry after some time. Avoid drying it in the dryer, because it can melt. Repeat this step once or may be twice.
Wring or squeeze the Coat side to side
You can always soften your coat by spreading the coat on a dry towel and then rolling up the towel and mildly squeezing it from side to side, irrespective of whether the coat is dry or wet. Repeat this process a few times and then unroll the towel.
Brushing the coat
Brushing is yet another technique to soften a plastic raincoat. So, brush the coat with a hairbrush but ensure that the brush only has plastic threads and not metal, otherwise the coat will be damaged. Focus on brushing the arms as they are in motion all the time.
You may repeat any of the above mentioned steps till it gets as soft as you want. The coat is now ready to be used.