Well, from my own research into it, (I'm a nursing student) you need at least 3-5 years of ER/Critical Care experience, ACLS, NRP, PALS, all the applicable certifications are preferred as well as certification as an emergency nurse (CEN), critical care nurse (CCRN), or flight nurse (CFRN). Also, I hear that some companies prefer experience as an EMT or EMT-P. They also like to see instructorships in the previously mentioned certifications (ACLS, NRP, PALS, etc.). Seems like an exciting career. I hope my path takes me in that direction one day. Best of luck.
Edit: I realized I didn't answer the question about training. I believe most of the advanced training (intubations, central lines, etc.) is done after you are hired on at a flight program. I may be wrong about this though. Also, if you are interested, there is a Master's program with a focus on flight nursing at Case Western Reserve University. http://flightnurse.case.edu/templates/index.htm