  Using a regularization by putting the system in finite volume, we develop a
novel approach to form factor perturbation theory for nonintegrable models
described as perturbations of integrable ones. This permits to go beyond first
order in form factor perturbation theory and in principle works to any order.
The procedure is carried out in detail for double sine-Gordon theory, where the
vacuum energy density and breather mass correction is evaluated at second
order. The results agree with those obtained from the truncated conformal space
approach. The regularization procedure can also be used to compute other
spectral sums involving disconnected pieces of form factors such as those that
occur e.g. in finite temperature correlators.
