All of the internet providers are changing their policies for residential customers who constitute the heaviest users of system bandwidth. What's being implemented is sometimes monthly caps, sometimes added charges, sometimes both.

If you're now over 300 GB per month, I'd guess in the future you will be using it less and paying more for what you use. You're probably already well aware of what your household's most data intensive uses are and who's responsible for such uses, the time may have come to start cutting back. Or, if you can afford it, you may need to just bite the bullet and understand your bill will be increasing.