Clips on iPod cases work fairly well if you attach it just right to your clothing, and you're not jumping around too much. But your iPod can go flying if you move too fast. And some clothing doesn't work well for clipping an iPod to (for instance, the neckline of a sundress or the waistline of a bikini bottom would not look attractive weighed down in that way).
iStik cases fasten to your clothing through magnets. In addition to the case, there's a magnetic backing that you place underneath your clothing to hold the case in place. According to the iStik website, the extreme magnetic attraction created by the eight Neodymium magnets in the iStik case ensures that it stays in place while you're riding your bike, running, doing yoga, and other sports.
The iStik for 3rd generation iPod nano is priced at around $27 at Amazon. The nano 2nd generation case is $25 at Amazon.