This is VERY typical and normal at this age.
I am not saying it's OK, or easy to get through as the adult who has to deal with it, but rest assured it's normal.
Go to the site www.about.com and search on the phrase "your eight year old child" and look at the article there on "emotional development" for this age. The actual link is extremely long and I'm not sure it would post here. The article will let you know why she is acting this way!
Eight is one of the transitional ages for kids. Around the age of eight, kids are moving out of being a young child and starting the path towards puberty-- yes, puberty, even though they seem too young for that. You may feel like,"She doesn't act like a preteen, and I see no changes to her body," but believe me, this is the start of it all and she is not just being individually pouty.
I have a girl of 13. I cannot tell you how very many times, around this age, the moms of all her friends would say, "Is your daughter really sensitive and easily upset over nothing? Mine is, all the time, and I never know what might set her off into pouting/crying/being angry/stamping off to her room for no apparent reason." So you are not alone, and this is not just you kid being this way. Does it help to know that?
By about eight their bodies are starting to change and their brains are too, and they have much more complex mental lives at eight than we realize. They also are trying to "do it for myself" and start to push us away a bit -- and at the same time they are still kids and need us, though they may resist needing us too! The result: Poutiing and not telling us why, and not being satisfied no matter what we do to accommodate.
That's why, to me, unless she is really grumpy and disrespectful towards you and others as part of the pouting - it's not a basis for discipline but something you can choose to ignore. You asked how to manage it, and a lot of managing it is letting things go! This is truly, truly a case of "choose your battles" and it's not worth fighting the fact that she is touchy and sensitive; she actually can't fully help being that way though it may seem to an adult that "She could choose to be more cheerful, she could choose to stop pouting." Not entirely, not at this age or at some stages to come. I am not saying to cut her slack if she is mouthy or sassy or rude, but I do say ignore whatever you can ignore. Offer one "modification" that is reasonable and if she says never mind, then drop it.
Our school counselor then and now, with larger issues, advised giving a kid space to BE sensitive and grumpy. Acknowledge it and don't judge. Make statements (if you have some idea why she's being that way) like: "I can see how you'd be worried about that" or "Wow, it does sound tough" or things that show you are recognizing her feelings, and don't always try to solve things or have her "come up with answers." She doesn't really want answers, she wants to....be grumpy, for a time, and just needs to know you're there.
Yes, do take things away or tell her to leave the room take some time by herself if she is being annoyingly pouty and can't move on from it. Absolutely. And distracting her from her pout (like you'd distract a toddler who is unable to stop fussing!) can work. But short of rudeness or lashiing out that's not acceptable -- acknowledge, let go of what you can, and wait. This is HARD to do. I still have a hard time remembering to do it with my young teen and often kick myself for not remembering, "Step back, don't try to solve things or tell her what to do, acknowledge and don't engage."
You may get comments on here about hard discipline and crushing her for this but it will make her worse over time, especially if this is general over-sensitivity and not some specific behavioral issue.