Tips for designing PCB
I'm not sure what you mean by unconnected tracks. If you mean unconnected copper, you are right, they will capacitively couple so you should either remove them or connect them to ground.
Ideally, you would do a multilayer board and have one layer be a ground plane. However, you may not have that luxury and, frankly, for a 20 mhz PIC design it isn't necessary. I try to have all unused areas on the PCB have grounded copper. It's a good idea to try to have a "star" configuration for the ground - i.e. avoid differential ground path lengths. But, again, your boards will probably not be that critical.
a quick few other tips. use lots of bypass caps, isolate noise producing sections from clock, analog and digital signal paths, don't parallel long signal runs.
If you are making the boards yourself, don't forget that you won't have plated though holes.