It depends on the level of quality of the show you want to see. There are lots of ways to get cheap seats - it just depends on what you're willing to endure.
1) Most, if not all, shows have lottery tickets. If you show up at a specified time in the afternoon (cash in hand), they'll lottery off the front two rows. That information can be found: http://www.playbill.com/celebrity...y-Policies
2) TXTS is a booth where you can get half-price tickets for Mezzanine seats for most shows. Booths are available in several boroughs. Information can be found here:
3) If you want a C / D rated show, or one that's just emerging, I suggest checking out www.goldstar.com
They frequently have very good prices and reasonable deals for emerging shows. I got tickets to If/Then (the new Idina Menzel play) coming up soon.