A Transzorb is a good, low-voltage varistor. This keeps the Vcc from ever going above 7V.

An "LF" part is one that has LF in the part number and can run at 2V (at 4MHz).
For example: Instead of using an 18F8722, use an 18LF8722. Read the last pages
of your datasheet.

If you feed the Vcc power through a Schottky (a Schottky is a low forward voltage drop diode), then negative-going disturbances on the power supply line will not affect the PIC.
The capacitor on the PIC side of the diode is not only to add filtering, but also to provide power to the device during short negative-going transients. Since the Schottky diode has a drop of approx 0.5V, if you start with a
Vcc of 5V, after going through the diode, your PIC will only get 4.5V. Using an LF part assures that it will still be running within specifications.

The R/C network on the inputs not only protect, they help prevent false triggering of inputs
by narrow spikes.
You usually can't put much resistance in series with an output, but 330 ohms does provide a fairly good compromise.

I use the techniques for military devices, and they go through some awfully tough qualifications.