I've been finding it really helpful using lattices to sculpt blendshapes recently. They give volume control and flexibility. That is we can change number of divisions of lattice and where lattice is positioned so we move just the area of face we want to.
I highly recommend you check out Chris Pagoria's work. Check out his Anim School demo on advanced rigging (Animation School - AnimSchool Classtime: Fast Start For Facial Blend Shapes ).