That merge at 8, by the Brooklyn Academy of Music, is a big deal as things narrow, although I haven't been there with the big crowds.
If it's a bit chilly, most people take a long sleeve shirt to the start, and discard it to the side shortly before the horn; they all go to charity. Some guys bring-wide mouthed bottles as well and discard them to the side beforehand as well. Long pre-race lines at the port-o-sans.
You have to fill your bag -- I had a lot of stuff in mine -- and then bring it to the appropriate UPS truck a while before the start. (The trucks are lined up along the Park Drive starting about 1/4 mile north of the finish line.) Err on the side of underdressing, as it warms up on the course. Put the other stuff, except for that long sleeve shirt, in the bag.
Take water at every stop; they come every mile after 2 (except on the 59th Street Bridge), with Gatorade as well every 2 miles. There is also a Gel stop at mile 18; if you train with Gels, remember the color of the label for flavors you've used and grab that one (for me it was the blue of vanilla or the red of strawberry; I got the blue).
Most important thing: From a sign I unfortunately did not see in 2006: "Take it easy, it's not a 10K." The crowds on 4th Avenue in Brooklyn are unbelievable and can really push you too hard without you even noticing (even when you're checking you splits). The first 2 miles will give strange splits because of the bridge and the traffic. Focus on pace when you hit 4th Avenue and take it easy.
I hesitate about giving this final bit, but here goes. If you need to stop late in the race to gather yourself, it's not the end of the world. A few seconds, walk a little, catch the breath, and start up again. No need to panic. In 2006, I did it at 21, 23, and 24.5. But, remember, I hadn't seen that "not a 10K" sign.
Good luck and have fun.
