  A classical particle oscillating in an arbitrary high-frequency or static
field effectively exhibits a modified rest mass m_eff derived from the particle
averaged Lagrangian. Relativistic ponderomotive and diamagnetic forces, as well
as magnetic drifts, are obtained from the m_eff dependence on the guiding
center location and velocity. The effective mass is not necessarily positive
and can result in backward acceleration when an additional perturbation force
is applied. As an example, adiabatic dynamics with m_|| > 0 and m_|| < 0 is
demonstrated for a wave-driven particle along a dc magnetic field, m_|| being
the effective longitudinal mass derived from m_eff. Multiple energy states are
realized in this case, yielding up to three branches of m_|| for a given
magnetic moment and parallel velocity.
