  A continuous sequence of infinitesimal unitary transformations, combined with
an operator product expansion for vertex operators, is used to diagonalize the
quantum sine-Gordon model for 2 pi < beta^2 < infinity. The leading order of
this approximation already gives very accurate results for the single-particle
gap in the strong-coupling phase. This approach can be understood as an
extension of perturbative scaling theory since it links weak to strong-coupling
behavior in a systematic expansion. The approach should also be useful for
other strong-coupling problems that can be formulated in terms of vertex
operators.
