  We use Heavy Quark Effective Theory (HQET) techniques to parametrize certain
non-perturbative effects related to quantum fluctuations that put both heavy
quark and antiquark in quarkonium almost on shell.
  The large off-shell momentum contributions are calculated using Coulomb type
states. The almost on-shell momentum contributions are evaluated using an
effective 'chiral' lagrangian which incorporates the relevant symmetries of the
HQET for quarks and antiquarks. The cut-off dependence of both contributions
matches perfectly. The decay constants and the matrix elements of bilinear
currents at zero recoil are calculated. The new non-perturbative contributions
from the on-shell region are parametrized by a single constant. They turn out
to be $O(\alpha^2/\Lambda_{QCD} a_{n})$, $a_{n}$ being the Bohr radius and
$\alpha$ the strong coupling constant, times the non-perturbative contribution
coming from the multipole expansion (gluon condensate). We discuss the physical
applications to $\Upsilon$, $J/\Psi$ and $B_{c}$ systems.
