I feel your pain. My DD 21 months can be quite bossy, and just imagine when she gets to talk more and express herself better. We did start time outs with her and it does help. I don't use it a lot but it does help. When she is doing a behavior I also tell her that she needs to stop or she is going to time out. This usually stops it. If it doesnt, she goes to her crib for a minute. That is plenty of time for her. I bring her back and try again. Other things I do is I don't argue with her, if I say no once, that means no. I try to distract at times, but if that doesn't work and she lays on the floor and throws a tantrum, I move away from her (obviously making sure she is in a safe place). It doesn't last long, it's all about attention and control for her. Also, if she keeps changing her mind and being demanding I don't give in to it. Last night she wanted a cookie (because she had gotten one the night before for a treat). You should have heard the way she was saying "I. WANT. COOKIE!" It cracked me up the way she paused in between. Thankfully I don't laugh about it in front of her..but I do chuckle with DH at times when she doesn't see. Don't want to confuse her too much.
Good luck, it will eventually pass I am pretty sure. DS4 wasn't quite this bad but he has his moments..but since he is older they are few and far between.
