Andrew, this is what I do to minimize warping and curling. First, as some of the others have suggested, I use a non-water based stain, usually black alcohol. Also, I stain both sides of the piece as that decreases the tendency of wet grain raising and pulling against dry grain. Then I place the wet pieces between a couple of paper towels and weight the whole thing for 12-24 hours with something heavy, usually a stack of books.
Finally (and I know this sounds crazy, but it always works for me), if your siding is still curled after it's dried, iron it with a warm, dry iron. I set mine on the "nylon/silk" setting, with the water emptied out and steam set to "off." I then iron the siding until it flattens and stays flat - usually about 30-90 seconds. Be sure to keep the iron in constant motion so that all of the residual moisture in the wood is evaporated without scorching the wood.
I even used the ironing method on two board and batten roofing panels that curled slightly after I had glued the battens on. Since the roof was to be removable, gluing it down was not an alternative. After 2 or 3 weeks, I still couldn't get them to unwarp, so I put the iron on them. They flattened right out and have remained that way for well over a year now.