How can I fix a warped canvas?

A canvas that I already painted on is now warped diagonally. It is a 36 x 24 inches canvas and had only one support on back... I am going to show this piece at an exhibition in few weeks. Please give me ideas of how to make it straight. !!
Update: thx everyone for tips. I will try Clare's way first, and will re-stretch it if I can not do it well.

one question for Jacob- do you mean paint X the back on the canvas(fabric)? then the stretcher bars won't warp? Or do you mean when you work on wood panel?
4 answers 4