Toughing it up will be your best bet. I didn't do that, and college has forced my interests towards other things. If you are to go up as a civilian, your best bet is to select a field of research that somehow involves low-gravity experimentation. I had no interest in low-gravity experimentation, only floating around like a kid, so my studies led me away from that direction in college. In the Air Force, I presume the track is much more rigid.

Selection for being a NASA astronaut is also extremely competitive - I've heard percentages of applicants accepted to be under 1% (but I can't back that up with concrete sources, or I'm too lazy). It is dependent on far more than mental aptitude - you have to be a physical prowess to be eligible. (That definitely struck me off the list.)

What can you do now? Math is the only definitive answer I can provide. You could be investigating anything from microphysical phenomena in low-gravity, or biosystems development in low-gravity. Bio-type excuses are the easiest for me to think of, since that's really not been done much.

Best of luck in achieving your goal.

Oh yeah, it's far more serious work than media presents it to be.