As I ponder the interesting tips and tricks I've learned through my extensive research of (ahem, addiction to) home improvement shows on the likes of HGTV and TLC, I can think of no tip that has worked better than one that I saw on BBC's "How Clean is YOUR House?"
To keep your bathroom mirror from fogging, apply a small amount of shaving cream to the entire surface, and wipe (don't wash) it off. Your mirror stays moisture free for about a month (unless you wash or wipe it clean frequently).
I have personally tried this and can't recommend it enough; probably to the dismay of my friends.
Any other home improvement show addicts out there who have a favorite tip or trick they've learned from a "few" hours of watching?