Just say no
Everything I've read and heard and been told has strongly advised me to NOT say "no" to my child. Pediatricians, child psychologists, the Internet--you name it, these folks all say no to "no." My husband and I joke about this constantly because it's nearly impossible.
From what I understand, "no" is only to be used when there is danger present--such as your kid preparing to tongue a light socket or touch a hot skillet on the stove. "No" is not to be used for the ten thousand other things your kid is constantly doing now that he's super-mobile. I guess saying "no" simply teaches kids to say it right back to you even sooner.
"No" is hard for me to let go of. I have two dogs, and "no" is a big part of their lives. "No," in fact, works pretty well with dogs. It doesn't always work for babies though. Mine just looks at me when a "no" slips out and usually smiles or laughs and/or continues on with whatever he's doing that I'd rather he not.
My husband laughs about what sorts of things we're supposed to say instead of "no." He asks if we're supposed to say things like, "I'd really rather you find another activity than continue to pull all my hardback books off the shelves." At least, lengthy as that is, it would teach our son more vocabulary.
I try not to say it, but occasionally it slips out anyway. Then I feel sort of bad. Then I get over it. I try to pick the kid up, move him to another area and another activity and hope for the best. Ten times out of ten he's back doing what he is not supposed to be doing before I can even sit down. So, sorry, but sometimes he gets a firm "no." And sometimes it actually works.
Any ideas what else we might do to avoid using this old standby? I'm tired of hearing myself say--or think--the word "no!"