We give explicit sequences of strong and fast (``bang-bang'', BB) pulses that create the conditions allowing for the existence of large decoherence-free subspaces (DFSs). One such example is the creation of collective decoherence, wherein all system particles are coupled in an identical manner to their environment. The BB pulses needed for this are generated using only the Heisenberg exchange interaction. In conjunction with previous results, this shows that Heisenberg exchange is all by itself an enabler of universal fault tolerant quantum computation on DFSs.