1. Instal Java JDK in your environtment

 

2. Download the YUI Compressor JAR

The YUI Compressor is written in Java and distributed as a JAR. Download the JAR from the Yahoo! site and save it to app/Resources/java/yuicompressor.jar.
Unzip, go to build/ dir and extract yuicompressor-2.4.7.jar to put it in your app/Resources/java/ project dir. (In addition, you can rename yui file for yuicompressor.jar)

3 - Enable yui compression for prod environement

This is logical to add yui compressor in your config_prod.yml file and not for all environment (you should remove lines from config.yml).
So add theses lines in config_prod.yml:

assetic:
    debug: false
    filters:
        yui_js:
            jar: "%kernel.root_dir%/Resources/java/yuicompressor.jar"
            apply_to: "\.js"
        yui_css:
            jar: "%kernel.root_dir%/Resources/java/yuicompressor.jar"
            apply_to: "\.css"

4. Verify you prod controller file

Go to web/app.php, and set prod / false (false for no debug).

$kernel = new AppKernel('prod', false);

5. Clear cache

If you want to rebuild your CSS and JS (after update), never forget to clear cache in prod otherwise modifications will be ignored by yui compressor:

php app/console cache:clear --env=prod --no-debug

6 - Regenerate fixtures with compression

you can generate and compress your files !

php app/console assetic:dump --env=prod --no-debug -v

Note that -v show more details about the process of compression, you will be able to see if there is an error taht causes problems with yui compressor !
For example, the JS comment tag "/!" causes trouble with yui compressor, you should rename all "/!" for "/*" in your JS files.

7. Fixing Clear Cache Issue

because you had been clear cache, then you will have  problems on the path.
to overcome execute this command

rm -rf app/cache

And feel the different