How should I organise my CSS? [on hold]

I have been developing HTML / CSS (+ javascript) websites for some time now and I have come to the point where I feel I need an advice for structuring the code. I don't have any problems with organizing HTML files. Yet, the CSS part is much more complicated. I have begun using media queries thus ending up with files with > 1000 lines of code. How can I improve my coding so the files would be more readable, cleaner and smaller in size? Do you any tips that could help me structurize my code?

Thank you!