I change my clothing quite frequently! Often I will anticipate, especially if I am familiar with the terrain. At the bottom of a big climb I will pull off a layer or two. Then at the top, before the big descent on the other side, I will add a layer or two. Keeping some dry stuff stashed safely away to pull out at stops, that is another smart strategy for long rides. Of course, need a good waterproof layer to protect it at the stop! Could use something like a sil-nylon poncho at that point. Then stuff it back in the saddlebag or whatever when getting back on the saddle.