This is a numerical study where we propose and quantify a technique for increasing the efficiency of convective systems. The concept is based on "trimming" or "shaving" and locally discarding the warmest part of a developing thermal boundary layer such that fluid at moderate temperatures is always absorbing thermal energy from the heated surfaces, which are not allowed to overheat. For the present study, a parallel plate channel that is subjected to an internal forced convective cooling is the configuration considered. Therefore, geometrically speaking, the proposed trimming technique simply consists in optimizing a blunt reduction of the spacing between the upper and lower plates composing the channel along its axial location, such that the new configuration has a step-like geometry, allowing the heated coolant to be discarded at the transition between the two sections. The numerical results, which are directly compared with the performance of an optimally spaced flow between two parallel plates, show that the thermal performance can be increased by over 20% with two levels of trimming along the flow axis for forced convection i.e., three sections with different plate-to-plate spacings. The numerical predictions are also compared with scaling-based results showing a good agreement. Additionally, numerical simulations also considered buoyancy driven flows (i.e., natural convection); however, no significant performance gains were observed when the boundary layer trimming was used in that case. (C) 2013 Elsevier Ltd. All rights reserved.