ommmward, my older two are 18 months apart... i never really had an opportunity to learn to deal with dd's personality one on one before ds1 was in the mix.
both of them are stubborn, exptremely so. and both with happily cut of their nose to spite their face. dd would rather wet herself and stay in wet clothes for 3 hours than go to the toilet, so telling her she'll go to school in pj's really wouldn't get any response at all... she'd go happily in them and not even care, which would be fine except the school has a uniform and it's hardly teaching her anything to let her know she can break the rules whenever she likes but the other kids can't
and now there's ds2 as well and if i'm doing something with him (feeding/changing/bathing)i simply can't track the older two down to chat to them about their behaviour... sometimes i do just have to yell from the other room and if they don't respond i have to use things like countdowns and threats to get them to stop attacking each other/breaking something/playing with water and soaking the whole house/attacking the cat.
and on top of that dd is a pathological liar, she'll stand there with a crying ds1 in front of her, a stick in her hand, knowing full well i saw her hit him with the stick and say in the most indignant tone 'mommy, i didn't hit [ds1] with the stick'... how on earth do i reason with that?!
i'm definitely going to implement some of this starting right away, i just don't think certain aspects of it would work with our lot. and convincing dp to stop saying 'good girl/boy' as a verbal tic around them might be more than i can manage... the amount of times i've had to turn to him and say 'fgs, she's just slapped ds1, broken that toy and thrown a huge tantrum in the last 10 minutes... she's not a good girl just because she did what you asked that time!