/diy/ - Do It Yourself

Hey guys. I've built a deck and pic related is exactly how the railing looks. I haven't put the top 2x4 horizontal piece on yet but I want to maybe put a 1x6 deck board on it or maybe a 2x4 but how do I prevent it from warping? I've done several of these with friends and it seems they always seem to wrap to all hell once they dry out. Would a deck board warp less than a 2x4? Warp more? What about a 2x6? All of the ones I've helped on looked great when first installed then a month later are twisted all to hell.

Thanks for any advice