We study model-based feedback control of the low-Reynolds-number flow over a flat plate at large angles of attack, in both two and three dimensions. Our long-term goal is to be able to manipulate the leading-edge vortices that form on low-aspect-ratio wings at high angles of attack, and that often contribute to exceptionally large lift coefficients. In two-dimensional simulations, we present a model-based feedback controller that uses an observer to reconstruct the entire flow field from velocity measurements at three locations, and stabilizes the flow at an angle of attack for which the natural flow state is periodic shedding. In three-dimensional simulations, we use open-loop forcing to study actuator placement, and conclude that trailing-edge actuation is more effective than leading-edge actuation in influencing the forces on the plate, as well as the wake structures. Finally, we present initial results towards extending our model-based control design to the 3D setting, and apply a selective frequency damping method to find unstable equilibrium flow fields in 3D simulations.