  We provide detailed arguments on how to derive properties of generalized form
factors, originally proposed by one of the authors (M.K.) and Weisz twenty
years ago, solely based on the assumption of "minimal analyticity" and the
validity of the LSZ reduction formalism. These properties constitute
consistency equations which allow the explicit evaluation of the n-particle
form factors once the scattering matrix is known. The equations give rise to a
matrix Riemann-Hilbert problem. Exploiting the "off-shell" Bethe ansatz we
propose a general formula for form factors for an odd number of particles. For
the Sine-Gordon model alias the massive Thirring model we exemplify the general
solution for several operators. We carry out a consistency check for the
solution of the three particle form factor against the Thirring model
perturbation theory and thus confirm the general formalism.
