Lindsey and Kris, at 8 mos. old, the only tactic I used was redirection- redirecting my son's attention to something else. Fortunately, at that age (which I can barely remember) babies' attention spans are quite short. I'm not even sure when it was that my son understood the word no. As Heli mentioned, try to not get in the habit of saying no. It's difficult, but whenever possible I try to say things positively.
OK. I just read Heli's post again, and I've basically just totally reiterated what she said. But that's just what I did.
Oh, I just remembered- when my son was old enough to understand (?2+ ?), if he was doing something that could hurt him, I'd tell him, "That's not safe. You could get hurt." He understood that.