To soften my Workmans Utilikilt I just wash and then put it in the dryer on no heat. The tumbling softens it and if you take it out when the cycle is done it doesnt wrinkle.

Its the thing I've found that works best. After a run in the cold dryer the material is about as soft as the average pair of heavy jeans. Adding any sort of heat though, either from the dryer or an iron will stiffen it back up though.