My personal opinion is that the more physics you know, the better off you will be. Other than that, make yourself interesting. It might not help you get the job you want, but your life will be a lot more fun.
Patience. If you’re lucky, it takes at least 5 years to go from mission inception to getting back data. I’ve been working on trying to get a particular mission (a multi-lander mission for seismology to explore the deep interior of Mars) flown for 15 years now. I’ve been really close on three different occasions, but not quite close enough. It’s not easy persevering, but sometimes that’s what it takes.
Scott Carpenter went into space when I was in kindergarten, and I wanted to be an astronaut from that moment on. What I actually wanted to be was a scientist in space, which as it turns out isn’t really what an astronaut is, but that’s just a detail.
I actually gave up on being in space exploration when I was in high school, figuring that it was not a practical goal. I was going to be an engineer when I started college, but found physics (which they didn’t offer at my high school) incredibly fascinating and engineering classes incredibly boring. I switched to physics until I finished my degree. At that point one of my professors convinced me to go into geophysics, which sounded cool to me (I thought that I would be able to work outdoors, which was completely wrong). While in graduate school I met by chance another student who had worked at JPL for a while, and he convinced me to sign up for a summer job working for a scientist there. I got the job, and have been at JPL ever since.
I love to travel, hike, backpack, and listen to music of all kinds (classical, rock, jazz, punk, ...). I would be a sports car buff if I could afford it. I have a black belt in Volkswagen repair, having once thrown a rod in the middle of the Utah desert, finishing a complete engine overhaul (including finding parts for the relatively rare ’66 bug engine) and getting back on the road in just under 24 hours.