You didn't specify how long you've been playing. Is "a while" a week? A month? A year? Great flute tone does not happen overnight. Great tone is based upon building the right foundation on fundamentals. Have you studied with a private instructor who can see and hear you and advise you where your weaknesses are? You can only teach yourself up to a point and then you need a tutor to help guide you over the humps.
Great tone requires building a good foundation of breath support, lip flexibility, and lip strength. Anyone can run, but to run a marathon requires training to build the leg muscles, lung capacity, etc. Playing a flute is no different. You have to build the PROPER foundation.
The best thing to improve flute tone is to practice playing long tones for at least 15-30 minutes per day. Jennifer Cluff maintains a website with lots of good information about improving your playing. Here are some links to her pages about improving tone, playing long tones, and high note warmups:
http://jennifercluff.blogspot.ca/2012/0 ... -pdfs.html
http://www.jennifercluff.com/02%20Pure% ... armups.pdf