Yeah, connecting long flying wires directly to the inputs of the PIC usually isn't the best design practice for anything that's going to see some use in the real world. I am surprised though that it's picking up enough crap to drive the input high even with the switch pulling it hard to GND - that sounds a bit strange.

Provided that the switch is really switching "hard" to Vss I'd start with a simple cap (10-100nF) across the input. Then perhaps a little series resistance on the input (outside of the cap) but Watch out for the voltage divider effect with the pull-up. An inductor in series with the input lead (again on the outside of the cap) and/or a couple of turns on a ferrite core.

/Henrik.