At some point in time, I'll invest in the tools needed to swap out my cog. But until then, I'll keep relying on friends. Which means, I tend to run whatever gear I have for all my rides. Every 6-12 months, I'll make a change but I've noticed a disturbing trend.
The evolution of the gearing on my SS during the past 2 years looks like this:
At this rate, by March of 2010, I'll be running a 16t. I've heard the 16t cog comes with a discount on titanium knee joint replacement.