Here’s a neat resource I stumbled upon yesterday from The New York Public Library: Direct Me NYC.
Basically, it allows you to look through phonebooks from 1940 to figure out at what address your ancestors lived. Afterwards, you can convert the address into an enumeration district number, which makes searching the census a lot simpler. Whoever designed this website did a great job; it’s super easy to use. I’ve already found a bunch of my ancestors.
New York is in the process of being indexed, but Ancestry has already completed a bunch of states. Check out their list here. The great news is I’m pretty sure the images are free to look at too, even if you don’t have an Ancestry account. It’s a win/win situation for everyone!