There is a nice online tool cssminifier which has also an API which is pretty simple and easy to use.
I made a small python script that posts the CSS file content to that tool's API, returns the minifed CSS and saves it into a file "style.min.css". I like it because it is a small code that may be nicely integrated in an automated deployment script:
import requests
f = open("style.css", "r")
css_text = f.read()
f.close()
r = requests.post("http://cssminifier.com/raw", data={"input":css_text})
css_minified = r.text
f2 = open("style.min.css", "w")
f2.write(css_minified)
f2.close()
