This is just my 2 cents...There are certainly many approaches one can take in preparing for a trip to Nationals. Option 1: Go all out, do a bunch of events in the season, have a co-driver, prep your car to the max, etc. or 2) Do whatever you can reasonably afford (time, energy, and money) and simply get out there any way you can to see what it's like and to get the experience. Heck, if you have semi-decent tires, can get the time off from work, and can find someone to share hotel cost with, then go out this year. Then, when you go out next year you'd know what to expect (e.g., surface/grip level, format, pressure). One doesn't go to Nationals the first time to kick ass and take home a trophy, you go for the opportunity to learn, drive great courses, benchmark yourself against the best drivers in the country so as to measure your future progress, and have fun.
I started autocrossing in 1994 and went to my first Nationals in 1997. Driving onto Forbes Field in Topeka for the first time and walking the courses was like literally like stepping onto hallowed ground for me. Note: Reason #973 why I'm highly disfunctional

I finished last in the Pro Finale and mid-pack (26th) in what was one of the toughest classes there. I was happy with my finish and felt like I drove well. Part of the reason for going to Nationals (or any big event) for the first time is to simply get acclimated to everything regarding the event, so the next time you go there is less "noise" and emotion in your head. Hence, once you become more familiar with your surroundings you can then focus more on your driving.
Part of the reason I'm going this year, even though I seriously considered not going for a variety of reasons, is to learn the new site and surface to better prepare me for the future events out there. Even though I've only run a few events this year and my car now appears to be outgunned in class, Nationals is still the best learning opportunity available in autocross and thus is hard to pass up. Plus, it's just fun to watch all the classes, meet new people, and hang out with your friends.
I agree with Jim and Aaron's comments on how best to prepare. Since it's not always possible to do all that, I just wanted to throw out a different way to look at it. FWIW, there is no doubt in my mind that I had a lot more fun the first year I went to Nationals and got beat pretty bad than any year that I trophied.
Eric