Sparkly, your skirt has to be long enough that your knees will not show AT ALL in any position, whether you are sitting or walking or getting into a car. As I pointed out earlier, this means that that it HAS to be at least 4 inches below your knee, and most of the time a lot more. For a straight skirt, it has to be a lot more than that in order to ensure that your knees are covered AT ALL TIMES.
I think that some girls may be nechshol BECAUSE of the 4- inch rule. Instead of checking if their knees are covered at all times, they assume that as long as their skirts are 4 inches, they are okay, and that is not necessarily the case.
One pitfall that I have noticed is that sometimes girls/women will buy skirts that cover their knees as long as they don’t cross their legs, and they think that they will never cross their legs so it will be okay, but then without thinking, they forget and cross their legs.
Another pitfall is that some skirts are fine until you get in a car and start driving.
Another pitfall is that getting into cars makes your skirt go up, so you have to make sure that your skirt is long enough that your knees do not show when you get into a car. (personally, I never let my friend’s husband give me a ride unless I am wearing a REALLY long skirt, because when he drives me, I have to sit in the back seat, and the back seat of their minivan has a very high step).
Another pitfall, IMHO, (and I know that there are people who will disagree with this) is that nowadays it has become fairly widely accepted that most people wear stockings/tights as opposed to knee-his. I personally feel that people are sometimes less careful about letting their knees show because they think that it’s not such a big deal because they are wearing stockings. I wonder if they would be more careful if they were wearing knee his. The counter-argument that some might have is that some might feel that it has become minhag hamakom to wear stockings. In any case, stockings or not, you have to make sure that your knees are covered at all times. In Rav Bodner’s sefer on hilchos brachos, he writes that the difference between your knees showing with or w/o stockings is that if you are wearing stockings, a man does not have to turn around to make a bracha, he only has to close his eyes or look down (I hope that was quoted accurately – I don’t have the sefer on me – but I do know that it was a very small difference, and either way it is still a BIG problem).
Another pet peeve of mine: people who think it is untznius to wear long skirts, and meanwhile, they and many others wear skirts that are not long enough to make sure their knees are covered at all times. WEAR LONG SKIRTS, AS LONG AS YOU WANT – it is for sure one thousand times better than your knees showing, IMHO! (okay, MAYBE, they shouldn’t go all the way to the floor – I don’t know- but even that is better than your knees showing).
End of rant.