Scalding hot water works well. Pour it over the wood and then immediately set it onto a drying rack of shims as mentioned next. Another option is to soak it in full strength ammonia and then lay it across a bed of shim strips to elevate the side up about an inch. Load the other side with shims and some light weights that hold the side flat. Allow to dry out for about 24 hours until you cannot smell even teh slightest whiff of ammonia.

The idea is that the ammonia softens the lignum that forms the walls of the wood's cell structure. But when it evaporates away the lignum is fully restored.

For obvious reasons you won't be a household fan if you do this indoors. So out in the garage with lots of airflow is the way to go. About a 5 minute soak should do the job. But pull it out and test for flexibility