  We develop the theoretical procedures for shifting the frequency of a single
soliton and of a sequence of solitons of the nonlinear Schr\"odinger equation.
The procedures are based on simple transformations of the soliton pattern in
the Fourier domain and on the shape-preserving property of solitons. These
theoretical frequency shifting procedures are verified by numerical simulations
with the nonlinear Schr\"odinger equation using the split-step Fourier method.
In order to demonstrate the use of the frequency shifting procedures, two
important applications are presented: (1) stabilization of the propagation of
solitons in waveguides with frequency dependent linear gain-loss; (2) induction
of repeated soliton collisions in waveguides with weak cubic loss. The results
of numerical simulations with the nonlinear Schr\"odinger model are in very
good agreement with the theoretical predictions.
