Lots of folks mention: taking their lunch, or anyway not going out to eat. I did the opposite: I go out to lunch every day. Gets me out of the office, out of the rut, meeting other people (never eat alone!) and working out relationships (business or otherwise). Then back to work, energized and with a fresh outlook. But of course I'm at a different place in my career, where the cost is something I gladly afford for th…
You can always take a packed lunch to the lunchroom, or to a park, etc.
As I said, its nice if you can afford it.