I've heard that what feral cats need more than anything is fresh water. They've a knack for finding food, and oftentimes others are leaving food out for them, but fresh water is over-looked more often. It's hard because depending on where you live you don't want to leave standing water lying around, but if you change it on a regular basis it should be OK.
Also, if you live in a cold climate take a look at building cat shelters. These are places the cat(s) can go into to warm up when it is too cold outside.