Can anyone help?

I have four children who are 10, 8, 6 and 1.

My 8 year old ( girl) is a real attention seeker. She wants attention constantly even if it means she plays up if she doesn't get it. I have no problems with spending time with her when she behaves, but not when she doesn't. I try and spend equal amount of time with each child ( though the baby is home with me all day).

As I basically said, if I don't give 8 year old attention straight away even if I'm giving attention to another kid she plays up. Eg me and my son (6) were playing Guess Who. It's only a two player game. She comes along and starts putting all the pieces down which really annoyed both of us. I said she could help me but she said she wanted to play, I said to wait her turn. The thing is I asked her before if she wanted to play first and she said no as she was already doing something.
Things like this happen a lot.
She's also quite domineering even towards her older sister who is two and a half years older. And with her friends at school. She's often coming home telling us her friends run off on her I think it might be because she's quite bossy and cracks it if things don't go her way..

Anyway, I've tried time-outs, extra quality time, taking privalages ( no TV, no treats etc). I yell, I try talking to her.. She isn't always like this but 9/10 she is.

Last night she was really good. We did a word search together and she was fine. Even spending time with her brothers and sister she was ok.

But when she misbehaves what can I do?