Wow. Everyone has pretty much covered everything. This will be my 4th York in 2 years. I also took my son (12) last year on Saturday in October, and wished I had gone on Friday - some tables covered until 10am and some folks packing up at noon. He did great with the walking. He asked to come this year, but I hesitate pulling him from school.
1) Read Clem's Primer - it helped prepare...
2) Prepare to be overwhelmed and amazed at the sheer volume of trains and the generations they span.
3) Know what your want. Make a list. Use pictures vs. text.
4) Do research before hand and know exactly what you are willing to pay.
5) You will see wild variations on price for the exact same item - example: 6464 Series I re-issue $185 in one hall, $650 in the next. By the time I realized the $185 was the one I wanted, it was gone. See #3 - buy it or it may be gone. There were plenty more for much less than $650, however.
6) I personally do the Orange Hall last. Figure most of the stuff there is "newer" and manufacturer focused and don't want to miss out on any "deals" on personal member tables.
7) I typically do a round in all halls, then come back to "shop", unless I see something that is a must-have at a good price. Last year I took a chance and kept coming back to one table looking at an item, once per hour for the last 3 hours. My indecision and the sellers desire get rid of the item saved me $100.
8) Check under the tables. I was looking for 3 well-used girder bridges, they were not on my list, and by the time I was on my way home, I forgot to check there.
9) I brought a backpack and a bottle of water. Plus some plastic garbage bags in case it rained.
10) Last April I went with 3 other guys, and one of them drove. We had no plans to meet until 4:30pm. I decided to by a 25lb. engine that I then had to carry around with me the entire time. Lesson learned - if you carpool, get a 2nd key to the car so you can stash your stuff.