The guys over at Modd3d recently put together an excellent article on how to overclock your DS Lite. Parts needed include: a GBAccelerator DS overclocking chip, 30 AWG kynar wire-wrap wire, and a low-wattage, fine-tipped soldering iron. Full instructions here. Video demonstration after the break.
Why would you want to change the speed of your DS? Unlike regular computers and some other game systems like the PSP, DS (and GBA) games always expect the handheld to run at exactly the same speed. When you speed up or slow down the clock, the game speeds up or slows down. This is great for making your games harder or easier