I have always included a Schottky diode down from board power to the PIC's Vcc, so that the ICSP header can never attempt to power the rest of the circuit. This allows programming under PICkit power only, which may be more convenient, even if the circuit normally requires more than 100mA, or has specific voltage requirements.
I realise this may not help OP, unless he wishes to modify the board. Also, if the programming pins are used for other purposes in the circuit, it may be necessary to prevent the !MCLR, PGC, and PGD voltages from propagating as well.