I've been trying the "sitting" and kicking method ever since OldSub4 described it a while back. The first time I tried it I was amazed at how effective it seemed. I try to focus on turnover more then anything, I think it is the key. Try to be able to get to the point where you can switch in and out of sitting and striding at a moments notice, as you would have to in a race.
Check out this video of the mens white race...sure Chelanga won but look at the kick that Heath produced, taking down about 6-8 guys in the last 150...he clearly "sits" down into his stride to get faster.
http://www.flotrack.org/videos/coverage/view_video/234177/75916-mens-white-race-course-record
I try and use visualizations to perfect the technique.
-the hips should always be forward, when sitting and kicking you just get a little "lower" in the stride (hips still as forward as possible)
-work the hip flexors more, really whip the leg through as fast as possible. This part takes time in developing, but it is the main way to incease turnover.
-work the hamstrings more too, part of the reason sitting makes you faster is that you can extend your leg farther behind you+more powerfully
-try not to bounce or bound AT ALL. These should be the smoothest you ever feel when running
-I think heel striking for some people during these is almost unavoidable. I'm one of those people, it just happens
-as mentioned above, try and focus on turnover. Not that 180 number mentioned by JD, that was for 3k and up. Try for 240 as a starting point, focus on picking up your foot from the ground and placing it down (gently, just in front of the center of gravity) as fast as possible.
-knees should not come up all that high. It's not a shuffle, but you want to focus on turnover
-keep the body upright (it is tough sometimes but flexing your core while doing these can help. try and stay relaxed though)
These can be tough on the hips...do not practice too much, twice a week is plenty, once you get it once a week is enough to maintain.
Of course, if OS4 says something different then take his advice over mine, those have just been my experiences over the past few months.