- keep your torso in vertical line, don't lean forward
- your butt should be in the same vertical line as torso (it shouldn't stick out)
- bend your knees
- you should land on the front of your foot (not toes, though)
- any pain on impact is a sign that your movement is not fluid enough. Pain is important because it indicates that your feet aren't landing correctly. You're not supposed to raise your feet very high (there's no point, rising them high only means the impact will be harder). Try to adapt your pace until you feel little to no impact.
Long story short, of course it's going to hurt if you run barefoot using the same technique you use while wearing shoes. The idea is that shoes block the pain, not the impact. For more info about technique, google for barefoot +technique
I run using this technique, and so far I'm fine. Presently I use light tennis shoes (with flat, thin soles - I figured out it's closest I can realistically get to running barefoot in areas where broken glass is not uncommon). I'd love to try something like Vivo Barefoot or Nike Free (minimalistic shoes designed to come as close as possible to running barefoot while keeping the social benefits of not being kicked out of restaurant), but they're generally not available in my size (EU 48). I've just ordered the biggest Vivo Barefoot (finally in stock) to try them out.