Misanthrope sumed it up there with the dropping the leg thing. Here's how I see it, after analysing the shit out of this:
The act of pushing the pedel starts with a very subtle movement of the upper leg and hip and travels down to your foot and to the pedal. Think of it this way, when you use your stick to hit a drum, the motion begins at your shoulder and travels to your stick in a very fluid motion and at the end of the motion you snap your wrist down. The bass pedals work exactly the same way. The motion starts at the hip, which drops the leg and then there is a very small and subtle snap of the ankle (like the wrist). Since the motion of hitting a drum or pushing the bass pedal involves the whole arm/wrist or the whole leg/ankle, very little effort is required to make it happen, the leg drop and the ankle snap work together in a single motion and when timed just right can produce a hard hit with almost no effort, meaning you won't get tired. Your heel shouldn't be very far above the bass of the pedal and you push with with ball of your foot on the pedals sweet spot. If it helps at first, you can try giving a bit of a forward motion to your leg as your heel drops.
You should practice and learn this motion, one foot at a time, for pretty long sessions, one foot at a time, without using your hands to distract you. It helps if you lean forward a tiny bit and rest some of your body weight on your snare to take some of the weight off of your legs cuz at first the tendency is to drop too much weight on the ankle from the leg and you just end up stomping the pedal without getting the ankle snap in there like an idiot (for a great example of how NOT to do this, take a look at Lar Ulrich in the video "One", what a fucking loser! hahaha). It's a fucking hell of a lot harder than it sounds but once you get it it's easy. The alternative is to keep trying to do it with improper technique and you'll probably never get it this way because it's such a subtle motion, at first it feels very unnatural so I don't think it's something many people will just start doing on their own without thinking about it and trying very hard, especially with both feet.
It takes quite a bit of time and effort to learn this technique but it's sooooooo worth the effort. Once you get it do yourself a favour and use only this techique for a long time, months and months, till it's ingrained in you, if you try to mix this heel up technique with heel down ankle snapping or just plain old mindless foot stomping like our buddy Lars then your legs will soon forget the subtle timing required to do this motion. Besides, this technique is so fucking cool and wicked that once you get it you won't want to use any other, except maybe out of frustration, but don't! Stick with it till you have perfected it, you'll probably feel like a shithead loser for the first week or 2 cuz it's so fucking hard, but it's worth it. Once you learn it then everything I've said here will make perfect sense.
I heard about that sitting high thing and I tried it and I think it's a bit of a cop-out. Once you find your "balance" and learn the motion well I think it's better to sit lower cuz then less effort is require to drop the ankle. Besides, all the greatest players I've seen, including those with the fastest feet, usually sit average to low so there must be a reason for it.. besides, sitting high is geeky, hehe.
Satori