Lots of good advice for a first-timer in this thread. 
I would add, that for modern Lionel/MTH/Atlas/etc., if the item was manufactured in the last 5 years or so, you will find the greatest concentration of it in the dealer halls, but for older modern, you will find a lot of it in the member halls, and often at great prices. I accumulated a lot of Atlas reefers and hoppers at $30-$35 a piece in the member halls, and haggling can sometimes get you even better deals, especially if you are buying multiple items.
Also, print a copy of the latest EDTCA fairgrounds map, or bookmark it on your smart phone, so that you can navigate the fairgrounds easier. I did this for my first trip to York, but did not on my second trip, and I almost missed a whole hall (White Hall). Fortunately, one of the announcements reminded me before I left, and I was able to take home some treasures I would have otherwise missed.
And I would like to echo that bringing lots of smaller bills ($5's and $10's) is a good idea for smaller purchases, especially early in the meet. As a table-holder, I once had the first 10 buyers at my tables just after opening buy single small items and pay with $50's and $100's and wipe me out of all my small bill change.
And finally, it bears repeating, if you see something you really want, buy it when you see it. If you pass it up, odds are very good it won't be there when you come back. I once saw something I liked, and passed it by, and decided I wanted it when I got to the end of the aisle. I immediately made my way back to the table (following the arrows, of course 
), and sure enough, it was gone.
Andy
