quantum mechanics - How does one make the wavefunction collapse into an eigenstate of a particular operator?

Say one has an unperturbed system that can be described with a wavefunction that is a superposition of many eigenstates. How does one make the wavefunction collapse into an eigenstate of say, the energy operator rather than say, an eigenstate of the momentum operator? In other words, what is the mechanism that decides into which eigenstate's operator the wavefunction collapses when a measurement is made?