Crucial spots for me included: immediately after the bridge to hand off my fanny pack, after the Queensboro Bridge (the longest stretch without any people), and the short bit in the Bronx. Knowing that there are going to be friendly faces up ahead is a great motivator, miles before you even see them cheering.
If you have people who are trying to cheer for you who you really want to see (like family or people who traveled into town), figure out ahead of time what side of the road they'll be on so you don't accidentally run right by them. The sides of the street get crowded, especially in South Brooklyn!
Even better than having people cheer you on is having someone jump into the race. This is heavily discouraged by the New York City Marathon, but if you have someone who has a bib from a past race and is willing to get yelled at, see if they can jump in around the Upper East Side. Miles 18 to 21 were much more bearable after a friend jumped in to run with us when we were losing steam.