WARNING!
System Update: Before using this or any other Minify Tool, ensure your CSS will work with it.
CSS LINT is what I personally recommend as it is helping me to get my code up to snuff to Minify.
Make a backup of your CSS file and do not save this over your only copy.
Copy your CSS Code and paste it in here.
Click the [Colapse and combine all!] button.
Once it is finished, it will be copied to your clipboard. Simply paste it into your .css file and link it to your page.