My feelings on getting the best turnout is to get your bunz out there and start promoting now.
Summer is still here. Start organizing more regular meets with your local members. Encourage people to meet up more often. It doesn't have to be huge events, just gatherings, getting to know each other, etc. (this is mostly directed at the newer members, or the old members that have fallen off the charts for a while)
If you've ever read through past threads, especially in the regional forums, you'd see that a lot of them were really tight groups.
They'd attend local car shows, do a dyno day, hit a car museum, do a car detailing day, go bowling, bbqs at each other's houses, etc. That can still happen.
It really works well to help build up excitement for SZ. And you can all caravan out in a large group. Even if you're timid about meeting new people at a national event, at least you'll all have your local buddies to fall back on lol.
The beauty of going longer distances is also dividing up the road trip, making a stop along the way. We've had people stop at each other's houses and then caravan over together the next day.
If it's a distance for me next year, I'll certainly be doing that sort of road trip, and am looking forward to it.