Go to ASTNA's home page - there's a FAQ there that gives some detail about it.
I'd echo what ENRN said except for the 10 months-1 year bit. Spending some time getting to really know critical care is worth it, and it takes more than a year to really get comfortable with critical care; most places want at least 3-5 years in CC. Certainly, you could jump ship every year and do a different unit, but I'd keep it in the back of your head to spend a few years in an intense CC unit (SICU, CTICU, etc.) and really get comfortable titrating drips, assessing critically ill patients after vent changes, interpreting bedside gases in your head, etc.
EMS experience was helpful to me, but not necessary. If an EMS license is required by your flight program, they'll typically help you obtain it after you're hired. Creighton has an excellent program for experienced nurses to do just that. Right now, I would seriously spend time making yourself into a kickass nurse instead of trying to be all things in all cases.
Certify in everything you can think of. BCLS, ACLS, PALS, NRP, TNCC/ITLS, CCEMTP, PNCCT, STABLE....all of the alphabet soup.
Weight limits - our program doesn't have a stringent requirement, but has job requirements like "Must be able to carry 50 lb. pack up 4 flights of stairs". Keep fit.
Another big second for reading/learning everything you can. Join ASTNA and AACN and ENA and read the journals. Ask the RTs everything you can. Listen at rounds and ask questions about what you don't know. Make it a habit to learn something every single day at work, and don't be shy about admitting you don't know something and asking someone to teach you. I found that at an academic center, the med students/residents know so much about the Krebs cycle and the parasympathetic nervous system and osmotic diuresis and all of those topics we gloss over in nursing school - ask them, and truly listen to what they're saying - I have not yet had someone be irritated about teaching me something I do not know.
Focus on making yourself a solid nurse with a good understanding of the patho/pharm issues behind major disease processes and you'll get there. Everyone knows the difference between the nurse who knows their stuff and the nurse who has stars in their eyes and just wants to wear a flight suit. If you hire on somewhere and plan to jump ship at every opportunity, just telling everyone, "Yeah, but what I really want to do is be a flight nurse..." - well, every hospital has That Guy, and nobody really likes him. Don't be That Guy.