I hate giving parenting advice too, and I'm kind of a new parent so please no flames lol. We have a DD3, we got her as a foster child at 21 months old and sometimes it's hard because we don't know her background. The best thing we've found that works for HER is what I call a "brown out" lol. If we're sitting around watching a DVD (for example) and she starts whining or throwing a fit, she gets a warning. If she keeps it up, it's a brown out. DVD goes off and I get up and do things I need to do around the house. I tell her when she's ready to talk like a big girl (or sometimes I say "in your big girl voice" lol) then we can talk, until then no toys and TV stays off. That is usually enough to get her to calm down and talk to me, and then I can reason with her. If not then she just stays in the living room and continues to throw her fit while we go about our business. I refuse to argue with her any more. If she says I'm mean or "I'm not her friend" I just shrug my shoulders. When she gets in that mode there is no reasoning with her and the only thing that happens is hurtful words get said from everyone. And all she wants is attention anyways, and by giving in to her at all, she basically gets what she wants. When we stop what we're doing to yell at her, pander to her, whatever, she is in control. And in my mind I'm not giving her what SHE wants until I get what I want lol. So now she can follow us around, scream yell, basically do anything but break things or hurt us, and we just ignore her. Even if she does go to her room and play with toys I don't yell. At first she'll do it just to see if I'll drop what I'm doing and follow her, but in the end it makes her calm down. Normally she just follows me around and will start saying the most outlandish things so that I'll pay attention to her. Sometimes it's hard not to laugh. "I'm sick mommy!" "I have an owie!" "The dog bit me!" Occasionally we remind her that when she talks in her big girl voice we will sit back down, talk to her, and turn her DVD back on. I get more chores done this way too lol. I do my laundry, the dishes, and the best part is, it just about always works! (I should knock on wood though, you never know when that might change!) As soon as she does "give in" and talk in her big girl voice and use her words, then "the lights" come back on so to speak and we turn our attitude around for her. We don't still act mad at her (or else she will have no reason to "come around" in the future). We DO have a talk about how what she did "wasn't nice" and we make her apologize. But she has to be calm first or she just can't be reasoned with, and if she doesn't have an audience, "the show" just isn't worth it lol. I will say that the first time we attempted this, it took over 2 hours for her to calm down. She didn't know what to do! I seriously didn't think it would work, but I kept telling myself that I can't cave now, she'll know that if she whines loud enough mom will cave! She FINALLY gave in and stopped her tantrum and started behaving better, and it has never taken that long since then. Consistency is key, they are always testing those boundaries subconsciously or otherwise.
If she throws a fit in a public place we typically do something similar. Let's say we're in Target and she wants to buy something and starts throwing a fit when I say no, I just start walking away. If she keeps it up I'm not afraid to leave the store either lol. Anyways her tempertantrums have decreased drastically in the last 6 months since we went from the attempted time outs to this method. It probably sounds like it happens all the time but really I would say that a dramatic fit only happens like once or twice a week now instead of like every night, and is typically MUCH shorter in duration. My husband will sometimes pull out the "you're acting like a baby and only big girls can meet the princesses" line lol! If she isn't all worked up it usually works too!

Seriously though I don't like to threaten things I can't deliver on. Every once in a while you have to hold up one of those threats just to show them you mean business lol. I will also say that in the beginning stages of a tantrum if she starts saying really ridiculous stuff for attention and isn't in full tantrum mode yet, I'll just kind of get this silly look on my face and look at her kind of crazy and say "Really?! Are you sure?!" and kind of start to smile. This will usually make her start cracking up and then I'll say "Oh I knew you were just teasing!" It's funny too cause you can tell she doesn't want to give in to the laugh but she can't help it hehe. If she continues the pout then I know it's going to get bad lol.
Good luck with your situation, I really feel for you. Each child is different and you have to find what works for them. Typically it's a control or power struggle issue, especially with mom's and daughters. The thing I found with too many punishments (or demerits of some kind if you're using a system) is that at some point when tensions are high and everything is heated, I would start taking away things or piling the punishments on one after the other, and then I think she would just kind of decide that she had nothing left to lose and she had no motivation to act better. And in the end all she wanted was my attention anyways, and I was giving it to her lol. This way we all take a moment to cool down, get rational, and not say things we know we'll regret later. I feel like my relationship has improved with her as well. It wasn't bad or anything, but sometimes I felt like we were arguing every night and she was starting to prefer daddy lol! Anyways my best advice is just to maintain the power/control of the situation and don't let HER "train" YOU if you know what I mean. When they start predicting your behavior and adapting their actions accordingly, that's when it gets bad. Those girls can really know what buttons to push too lol.
And just wait, one day our girls will be teenagers! Hugs!!
Val