As soon as wood starts to warp, move it to a completely dry location. Remember that moisture is the primary cause of wood getting warped. For this reason, storing it in a moisture free environment becomes critically important.
On a hot sunny day, take out the warped wood and place it in your lawn. Use a garden hose to water a patch of lawn, as big in area as the warped piece of wood. Mid morning is the ideal time to do this as the sun is just coming up in the sky.
Carefully place the warped piece of wood onto the watered patch of lawn. The concave side of warped wood should be in contact with the lawn. If the wood you are trying to straighten was excessively warped, only the ends may come in contact with the ground.
Allow at least the whole day for heat from the sun to set into the warped wood. As soon as the sun goes down, move the warped wood from the lawn into a shaded place such as your garage. Ask for help if the wood you are trying to straighten is heavy and cannot be easily moved from one place to the other.
Use an old towel to get rid of water particles clinging to the wood if necessary. At this time, the warping should be slightly reduced. You may have to repeat this process more than once before warped wood is completely straightened.