N00b-ish question: my car's coming up on 24k miles, and I've read a few stories of people with relatively low mileage and fouled-up spark plugs. So I'm wondering, typically how often do sparkplugs need to be replaced? What if I drive the car hard (lots of track and auto-x events)? Is the right answer simply for me to inspect them and decide? (And what do I look for?) And is there any advantage of getting aftermarket vs. OEM sparkplugs, cost or performance-wise? (I figure, a spark is a spark as long as it ignites the fuel.) Finally, how exactly does one "gap" a sparkplug?