My DD is almost 18 months, she is not a bad kid at all but I do find my DH and I are constantly saying NO. She loves to pull the DVDs off the bookshelf, throw toys, touch everything we tell her not to etc etc, basically all the things little ones tend to like doing. She knows she shouldn't do it as just before she does it she looks cheekily at us waiting for a response. We don't smile or laugh when we say NO but she still thinks its a game.
From what age does "time out" tend to work. I don't want to start shouting at her but when talking assertively doesn't work I have noticed the volume tends to creep up.