  We have measured the response of the flux-line lattice in the low T$_c$
superconductor, 2H-NbSe$_2$, to finite frequency drives. In a well-defined
range of fields, temperatures, and driving amplitudes the system exhibits a
variety of novel non-linear phenomena. Most strikingly, the flux-lines can move
easily in response to currents that are significantly lower than the DC
critical current if the direction of the current is reversed periodically while
the amplitude, I, is kept fixed, but they do not respond to a current that
periodically switches between zero and I, while the direction is kept fixed.
Pronounced memory effects associated with these phenomena indicate the presence
of dynamically generated structural changes in the flux lattice.
