Describing the large-scale behavior of weakly collisional magnetized plasmas, such as the solar wind, hot accretion flows, or the intracluster medium of galaxy clusters, necessitates a detailed understanding of the kinetic-scale physics governing the dynamics of magnetic fields and the transport of momentum and heat. This physics is complicated by the fact that such plasmas are expected to exhibit particle distribution functions with unequal thermal pressures in the directions parallel and perpendicular to the local magnetic field. This pressure anisotropy can trigger fast microscale instabilities -- namely, firehose and mirror -- which solar-wind observations suggest to be effective at regulating the pressure anisotropy to marginally stable levels. In this talk, I will use the results of weakly nonlinear theory and of new hybrid-kinetic particle-in-cell simulations to address how marginal stability is achieved and maintained in a plasma whose pressure anisotropy is continuously driven by a shearing magnetic field. [arxiv:1402.0010]