First off I am not a flight nurse....merely another wannabe. Wat I have been told (from several flight nurses) is the kind of classic path is as follows....
-approx 5 years of nursing experience split between ICU and ED. ICU for the critical thinking and exposure to those super sick patients. ED for volume, speed, trauma, getting good at assessments, triage, and physical skills. For the ED, I've heard Level 1 experience is best (for obvious reasons). For ICU, I have heard various opinions about which one is best and a pretty sound rationale for all of them. I would guess that whatever kind of unit it is, you would want it to have the highest possible acuity and exposure to different patients. I have also been told that getting some peds exp is very helpful.
-certifications. CCRN and CEN certs are very helpful in getting a job and you learn a ton while studying for them.
-courses. Obviously you have to have ACLS and PALS. Some programs require NRP. Most will put you through an ATLS(advanced trauma life support) audit or TNATC (transport nurse advanced trauma course) once you start working. Other courses that I have been told are helpful are TNCC (Trauma nursing core course) and ENPC (Emergency Nursing Pediatric Course....which you prob already have from working in the ED anyway), PHTLS (Pre-hospital trauma life support...especially if you do any prehospital stuff...more on that in a sec), CCEMT-P (Critical care emergency medical transport program). I have also been told that it is very worthwhile to become an instructor in ACLS and one of the other standard courses (TNCC, PALS, etc.).
-prehospital. I have been given advice both ways about getting prehospital experience ("yeah work on the street for the autonomy and to understand the prehospital setting and use advanced airway skills" and "No, you are a nurse so focus on nursing, you will learn the prehospital stuff once you start"), but you already have prehospital experience so the point is moot and I am sure that that experience will be an adantage to you.
Also, check out www.flightweb.com
for a lot more info about this ....just search therough the Getting Started forum.
Hope this was helpful.