It took me a couple of weeks (maybe 20-30 hours total) and thousands of throws to really "master" Ralph's combat throwing technique. The video he referenced was how I learned and it provides a foundation for you to work from. I use the term "master" loosely; by "master", I mean to the point where I could consistantly stick knives from any distance up to about 20'.
A couple tips for those who are still working on it:
-Use a large knife, I started with a 7" knife, and while I learned to throw it, I should have started with a larger knife (12-13.5"). That would have cut quite a few hours off my learning time I'm sure.
-Use a relaxed grip to start. Its harder than it seems, your instinct is to hold onto the knife just a fraction too long for the technique to work. You have to develop a feeling for how early to release, but for me that point was much sooner than I was initially comfortable letting go of the knife.
-I like a smooth, flat handle spine for atleast 4", although I've been able to throw much smaller than that. A large heavy knife, with a good flat handle (no crazy bumps or designs) of about 4" is my sweet spot.
-When you are just starting, throw it slow. Someone mentioned penetration being dismal. Well that won't last once you get the hang of it... Once you get the technique down (wrist, arm, grip), you will be able to start throwing it harder and you will be able to drive a big heavy knife really deep (so deep its hard to remove...).
Don't give up on this technique, its so useful once you get the hang of it you will be hooked on it.
These are just a few suggestions, Ralph is the real master.