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We derive an approximation to QED effects in strong background fields which can be employed to improve numerical simulations of laser-particle collisions. Treating the laser as a plane wave of arbitrary intensity, we split the wave into fast (carrier) and slow (envelope) modes. We solve the interaction dynamics exactly for the former while performing a local expansion in the latter. This yields a `locally monochromatic' approximation (LMA), which we compare with exact spectra for nonlinear Compton scattering and nonlinear Breit-Wheeler pair production, for realistic laser pulse durations. We show that, as an improvement over the commonly used `locally-constant field' approximation, the LMA exactly reproduces the correct low energy behaviour of particle spectra, as well as the position and amplitude of harmonic features.