A cap on the output, charge it up, then dump the charge. You'll get a negative spike, but that's about it.
If you read the datasheet for your particular PIC, you'll see under the 'Absolute Maximum Ratings' that negative voltages ( Vss - .3v depending on your PIC ) can damage your device.
Simple way - Charge pump - ICL7660/ICL7662 - using a pin on the PIC and a transistor or MOSFET to control that output.