From DIYinfo.org
A Cure With The Door In Place
It may be possible to cure a warp with the door in place, but it will be out of action for at least 24 hours.
1 Close the door, putting blocks of timber behind that part that normally rests against the frame. You will probably need to tape the blocks to the frame while you close the door.
2 Push the part that bows outwards back into place by screwing a length of timber across the frame. You will have to fill the screw holes later.
3 If the door bows out in the middle, put the blocks at the top and bottom of the door and screw the length of timber across the middle.
Taking The Door Off Its Hinges
If the correction cannot be done in place, take the door off its hinges and lay it on a flat wooden floor. Put blocks under the door in the places where the door normally rests against the frame, and correct the warp by forcing the door down to the floor with a length of timber screwed to the floor.
Adjusting The Door Stop
With internal doors, if the warp is not severe and is merely letting in draughts, a fairly simple cure is available. Prise the door stop away from the frame and re-fix it so that it rests against the face of the door. This will not work with exterior doors because the door stop is an integral part of the frame.