I'm working on some sculpting but I hate that I have to go to the Toolshelf and select
Subtract, whenever I need that brush. So, while in Sculpt mode, how can I switch between Add and Subtract brush modes, with a shortcut?
You can press and hold Ctrl to toggle between sculpt modes. While pressed you will be in the opposite mode to what is selected in the toolshelf; i.e. if you have selected Add, then while pressing Ctrl you will be in Subtract Mode.
A list of all keyboard shortcuts available in Sculpt Mode.
Ok, I found this: in the [opened-up] User Preferences » Input » 3D View » Sculpt » "+ Add New"... put "wm.context_menu_enum" in the first box; ignore the (auto-filled) "RNA" box; and put "tool_settings.sculpt.brush.direction" in the "Context Attributes" box. Now assign your carefully chosen shortcut and try it out... You should get a mini menu right under your cursor that offers a stable toggle; remote controls the Toolbox switch; & gives on-screen confirmaton (with brush icon color change) before you start sculpting. Nice!
I can't find a way to toggle without the menu though: the necessary basic commands "wm.context_cycle_enum" & "wm.context_toggle" both fail in my Blender 2.78a. If the menu irritates you, a way to zip past it is to disable shortcuts for the number keys 1 & 2 (usually sculpt brush select by index & general layer selection) & re-assign both to the Sculpt Add / Subtract shortcut... A quick double press on 1 or 2 then invokes the menu & the numeric position of the required item (Add or Subtract respectively).
Does this mean I get today's Blender nerd prize? :-)