We present an approach for the ferromagnetic, three-dimensional, and translational-symmetric Kondo lattice model, which allows us to derive both magnon energies and linewidths (lifetimes) and to study the properties of the ferromagnetic phase at finite temperatures. Both ``anomalous softening'' and ``anomalous damping'' are obtained and discussed. Our method consists of mapping the Kondo lattice model onto an effective Heisenberg model by means of the ``modified RKKY interaction'' and the ``interpolating self-energy approach.'' The Heisenberg model is approximately solved by applying the Dyson-Maleev transformation and using the ``spectral density approach'' with a broadened magnon spectral density.