Good advice, although from my experience (in San Francisco), getting to know the staff is the easiest way to ensure a free -> cheap working environment. Local places with a lot of people means you can get away with 1 - 2 cups of coffee for the day, which will run you $4 - $7 depending on how swank the place is.

Also, buy a couple of $5 power strips and leave them at the places you frequent. Give something back to the community that's supporting you (whether they know it or not).