It's been so long since I was at a National (and it was barely a big thing back then - 1988, I think). I did, however, go to several of the "Expos" in Toronto (Spring Expo, Fall Expo) which were really big. You just have to know what price you're willing to pay for which item. The chances of you "going back" to reconsider is probably not going to happen. However, if there is a dealer you want to revisit before you head home, write down the location and what item it was. (We always traveled with our checklist binder and extra paper in it.) Depending on which day you go: if you go early in the weekend, expect to find great, rare items, with big price tags and if you go later in the weekend (last day), you have a chance to score some bargains as a lot of dealers really don't wish to go home with that much stuff. Now, you won't be able to get them to sell you a Mantle lot for 10% of their asking price, but you may be able to get some good deals on more recent unopened stuff or factory sets.
Have fun!
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).