They say that attending live sports is dead and the average fan would much rather watch the game in the comfort of his or her own home, but I'm very much of the opinion that nothing will ever beat going to a stadium and cheering for your favorite team in person. Every fan has his or her own style, personality, and nuance when it comes to in-stadium fan support, and I think it's a blast to watch all of those fans come together as one big, happy, hydrated unit.
Personally, I have never been a big sign guy, but I certainly respect those who choose to take the extra time to go to the pharmacy, buy some posterboard, and increase their chances of getting on camera for a few seconds by writing something witty, usually via some phrase that uses a particular network's call letters.