Afrotechmods described in a YouTube video a way of having protection with a much lower voltage drop (and lower power dissipation). Basically it is this:
Of course, the input power doesn't have to be 8.4V, just enough to turn on the P-channel MOSFET (use a logic level one I would suggest). In my particular case I was using it to protect my robot car from a LiPo battery pack being connected the wrong way.
Depending on the current drawn, and the RDS(on) of the MOSFET, the voltage drop can be very low (eg. 0.1V).