Had the same issue on my 32Go surface 2. If you look in the PC settings app, you can see which apps are using which amount of data. Typically, one of the biggest contributor for me was the MSN News app. The cache can grow to more than 1Go and you have absolutely no control on it. I think this is insane, especially compared to Flipboard which uses less than 100Mo
Anyway, what saved me a lot of space is to delete the recovery partition. You can check for tutorials, it's rather easy. You have to save the partition to a USB key, and then you can save 4-5Go on your system.