We present estimates of the energy gain of highly charged ions approaching a LiF surface, based on a modified classical-over-barrier model for insulators. The analysis includes the energy gain by image acceleration as well as the deceleration due to charge-up of the surface in a staircase sequence. The role of the frequency-dependent dielectric response of LiF is emphasized. The resulting velocity dependent total energy gain is studied in detail and the results are compared with experimental data.