Assuming you're not referring to the 'soccer kick' that everyone else has brought up, but are instead referring to the FILIPINO scorpion kick in which you bend over so that your chest is parallel with your knees and you swing your leg up and basically kick over you head...
then i would say that first you need to figure out which leg is most flexible. which leg can you go more comfortably into a split (even if you can't go out all the way). My left leg split is the best, so i do the kick with my right leg (if you try to picture it in your head it's basically like doing a standing split).
Afterwards, the best stretches to do are splits (if you're kicking right, work left leg splits, and visa versa). You need to stretch out the hamstring of the standing leg A LOT...seriously, get it as loose as you can. The first time i started working the kick (which i do happen to view as being a REAL kick...and looks awesome as a board break) i didn't stretch and my hamstring is still hurting (and this was almost 4 years ago now...pretty sad). Also stretch out the hip of your kicking leg, stretch your back (lean forward touch your toes, also work backbends and bridges so that your back is nice and nimble).
I don't have any websites to show how to do the kick, because my searches in the past have been fruitless...
But this is basically how you do it.
if you're working with a partner your best bet is to have them hold a small target (one of the square ones), or, the best target to use is a Muay Thai kicking target ( http://www.centurymartialarts.com/portals/0/Images/Products/1034-LG.JPG )
Have them in front of you but off to the side (if you're kicking right, have them stand in front but off to the left). The target should be positioned over your left shoulder (if you're kicking right), and it should be angled.
Let's assume you're kicking right
again, the target should be right over your left shoulder, practically touching, you can reposition as needed, this is what works for me. Start with your right leg back, and swing your body completely forward, so that your chest touches your knee (if you aren't flexible enough, you'll have to work with what you have, and possibly work on getting more flexibility), simultaneously (spelling) swinging your right leg up and kick with your right foot. Depending on your positioning and how you throw you're leg you will either kick with your ball or heel of your foot (i always end up hitting with my heel). You have to throw your body forward quickly and with a snap. You might want to try doing it on a mat..just in case you fall to far forward.
I know that that probably wasn't the best explanation, but it was the best i could do. What really helped me figure out the mechanics of the kick was the Matrix. I don't know if you've seen any of the movies, but at the opening fight scene of the second matrix movie (i highly recommend renting it or borrowing it because it did help me with the kick) Trinity does the kick after she flips off the motorcycle. She does it exactly as it supposed to be done, and it looks beautiful.
It's really a great kick, and if you work on your timing you could use it in a fight (but not in self defense situation...the last thing you want is worrying about falling on your face) if you get the timing down. The person your fighting/sparring would have to be just off your left shoulder in order to land it, and that's assuming that you can do it quick enough for them not to react and sweep out your standing leg.
You said you do kickboxing, so i don't know if you would even be interested in this, but, it looks awesome in a demonstration as a board break, i never fail to get a few gasps when i do it :-)
I hope this helps!!
black belt...scorpion for 4 years :-)