I think that PhotoShop should have a backup command in addition to c-z and c-a-z. The idea is to "reenter" commands such as "curves" whose whole effect can be represented by a small amount of data (the coordinates of the points the user added to the curve for curves"). When the command is resumed, you are put in the state where you exited the command. You have a choice, in the current implementation, of either starting from scratch or performing the same command several times in a row. I think this idea of reentering a command would simplify my workflow and make it easier to conceptulize what I am doing.