The most popular dates, like Christmas Eve, and weekend nights, tend to sell out first, as well as the most popular restaurants. It's a good idea to be prepared with a date, and a back up or two, as well as a restaurant choice, and a back up or two, and call the first day they come out.
If you don't manage to get a package, I prefer to sit just outside the American Adventure to listen to the show instead of fighting the standby crowds. Last year, at least at the show I went to, there was hardly any standby seating at all.