Printed Circuit Board Design and Layout Questions

1. What are the primary design guidelines that must be followed when laying out a printed circuit board?
2. How do differential signals induce common-mode currents on the cables and structures attached to a printed circuit board?
3. How "balanced" does a differential trace pair on a printed circuit board have to be? (i.e. If the two trace lengths are within 1 mm of each other, is that good enough?
4. If I have to route a trace over a gap in a ground plane, what precautions should I take?
5. Should I always gap the ground plane between analog and digital circuits?
6. Does it matter if traces are routed along the edge of a PCB?
7. Is it a good idea to stitch signal return planes together around the periphery of a board?