How do I avoid overtraining?

First off, I'm not overweight or unhealthy - I just want to reap the health benefits of running, and I also enjoy the 'runner's high' experience when I'm done doing so. I took a break from running for a month or so because I had apparently been overdoing it, and could feel the wear-and-tear on my legs.

I pretty much want to run as much as possible (I just quit smoking and I'm in need of a healthy chemical high to replace the habit!), without injuring myself or making myself ache. Is there any way to train little by little everyday, or do I have to take entire days off from running just to keep my body out of harm's way?

Thanks in advance!