Part of the reason I've been able to see as many movies as I have this fall is free movie screenings. As you may or may not know, most major movies hold free screenings before their release as a way of building buzz for the film. If you know where to look (and are willing to wait in the cold), you can catch all the top movies without busting your budget. I'll take you through some of the main sites to find passes.
This blog's stated purpose is recycling, but it is hands down the best place to find out about free movie screenings in New York. It doesn't hold its own screenings, but it's a good central resource to find out about other sites' offerings. As it lists two to three movies a week on average, it's one to check regularly.
Fat Wallet Forums
This is a message board where free screening aficionados post codes and locations for screenings. It is not city-specific, so New York screenings are rare. But it's great for providing access to tougher sites like Gofobo and The Screening Exchange. It's also good for giving the links to studios' screenings, especially Fox Searchlight. My link above is to the newest page as of this posting, so you always have to click to the most recent comments.
This is one of the biggest free screening websites, and one of the worst for New York. Every now and then they get a screening, but it always fills up within seconds. If you're outside New York though, you might stand a better chance.
The Village Voice has a ton of great promotions, sometimes including movie screenings. If you sign up for their Movie Club, they'll e-mail you when they have one. It's been awhile since they've done a movie screening, but you can enter contests for dinners, concerts, and other events.
New York Observer
Similarly, if you sign up for the New York Observer's Cinema Club, they will e-mail you whenever they have a movie screening. And they do a lot of screenings.
Time Out New York
They don't hold many screenings, but it's easy enough to get a pass from the site.
Edge New York
This gay-themed New York magazine often lists screenings no one else has. They don't seem to have any at the moment though.
If you check all of these sites regularly, you're bound to catch a free movie a week. As for timing, you're generally let into the theater 45 minutes before start time, so: If it's summer, the movie is high-profile, and the screening is at a big AMC theater, you want to be there over 2 hours early. If it's cold or raining, the movie has no big stars, and it's playing at an art house theater, 45-60 minutes should do it. If the screening fits somewhere in between, use your discretion.