Origin of position-momentum asymmetry in quantum mechanics Elementary quantum theory teaches there exists a symmetry between position space and momentum space - you are free to switch by Fourier transform between position eigenvectors or momentum eigenvectors to express the wave function of a particle. 
In articles on decoherence I have read, but don't understand, that this symmetry is broken by environmental interaction and position becomes the preferred basis (or the preferred observable is the position of the system).
Have I understood this correctly? If so is there a simple way to understand the origin of this asymmetry between position and momentum (or an SE post where this has already been discussed at a simple level)?
 A: The asymmetry arises from the measurement apparatus. Which basis is chosen depends on what kind of environmental interaction you have.
In general this is formalised using the von-Neumann measurement scheme. It describes how a pointer gets entangled with the state variable, where the pointer is an approximately classical object.
The Hamiltonian that causes this entanglement to occur then determines which basis of the quantum system can be distinguished using the measurement apparatus consisting of the pointer and the interaction (e.g. the screen and the magnet respectively, in the Stern-Gerlach experiment, where the spin states become entangled with the position states on the screen).
Note that this only describes how the system and the pointer entangle, i.e. how correlations between them occur. From the comments below this answer I conclude that the OP's question is actually about how decoherence happens dynamically. The compulsory reference on this is Nieuwenhuizen et al., where they solve models that can describe real measurement processes. Please note that this does not have to yield the position basis as the preferred one, in fact in the particular Curie-Weiss model that is solved in the paper it is the spin basis again (simply because spin is easy to deal with).
