  We calculate the semiclassical magnetoresistivity $\rho_{xx}(B)$ of
non-interacting fermions in two dimensions moving in a weak and smoothly
varying random potential or random magnetic field. We demonstrate that in a
broad range of magnetic fields the non-Markovian character of the transport
leads to a strong positive magnetoresistance. The effect is especially
pronounced in the case of a random magnetic field where $\rho_{xx}(B)$ becomes
parametrically much larger than its B=0 value.
