My pitching coach used to say "throw every day"---even if it's just an easy game of catch, throw ten or fifteen minutes just to keep the arm free of rust. If you've been laying off for a few months, start slow, just to get the arm accustomed to the idea of throwing, then start to get more into it. I used to do that even in the winter, when I would find a venue such as a high school gym where I could do this without worrying about freezing my tush off (it does get pretty cold in New York), and I could always find a catcher so I could work on control and other stuff. The important thing is not to go all-out as if you were pitching the seventh game of the World Series; easy does it, take what time you need to get back into it. And attending a camp or two in March couldn't hurt.